ceph-salt barfs out a Traceback when Salt gets "Valid metadata not found at specified URL" error from zypper

When repos are not fresh (and sometimes even when they are), zypper occasionally throws a "valid metadata not found at specified URL" error. Salt and ceph-salt could handle this more gracefully than they do now:

```
master: Starting the execution of: salt -G 'ceph-salt:member' state.apply ceph-salt
master: [2020-04-30 18:28:47.269958] [node2.octopus-4n] [STAGE] [BEGIN] Distribute SSH keys
master: [2020-04-30 18:28:47.305720] [node2.octopus-4n] [STAGE] [END ] Distribute SSH keys
master: [2020-04-30 18:28:47.310786] [node2.octopus-4n] [STAGE] [BEGIN] Install and update required packages
master: [2020-04-30 18:28:47.367788] [node1.octopus-4n] [STAGE] [BEGIN] Distribute SSH keys
master: [2020-04-30 18:28:47.372835] [node3.octopus-4n] [STAGE] [BEGIN] Distribute SSH keys
master: [2020-04-30 18:28:47.375437] [node1.octopus-4n] [STAGE] [END ] Distribute SSH keys
master: [2020-04-30 18:28:47.378390] [node1.octopus-4n] [STAGE] [BEGIN] Install and update required packages
master: [2020-04-30 18:28:47.384565] [node3.octopus-4n] [STAGE] [END ] Distribute SSH keys
master: [2020-04-30 18:28:47.388160] [node3.octopus-4n] [STAGE] [BEGIN] Install and update required packages
master: [2020-04-30 18:28:47.700442] [master.octopus-4] [STAGE] [BEGIN] Distribute SSH keys
master: [2020-04-30 18:28:47.711976] [master.octopus-4] [STAGE] [END ] Distribute SSH keys
master: [2020-04-30 18:28:47.716377] [master.octopus-4] [STAGE] [BEGIN] Install and update required packages
master: [2020-04-30 18:29:08.774148] [node2.octopus-4n] Finished with failures
master:
master: Failure in minion: node2.octopus-4node.com
master: __id__: install required packages
master: __run_num__: 9
master: __sls__: ceph-salt.software
master: changes: {}
master: comment: "An exception occurred in this state: Traceback (most recent call last):\n\
master: \ File \"/usr/lib/python3.6/site-packages/salt/state.py\", line 1987, in call\n\
master: \ ret = self.states[cdata['full']](*cdata['args'], **cdata['kwargs'])\n File\
master: \ \"/usr/lib/python3.6/site-packages/salt/loader.py\", line 2030, in wrapper\n \
master: \ return f(*args, **kwargs)\n File \"/usr/lib/python3.6/site-packages/salt/states/pkg.py\"\
master: , line 1606, in installed\n pkgs, refresh = _resolve_capabilities(pkgs, refresh=refresh,\
master: \ **kwargs)\n File \"/usr/lib/python3.6/site-packages/salt/states/pkg.py\", line\
master: \ 944, in _resolve_capabilities\n ret = __salt__['pkg.resolve_capabilities'](pkgs,\
master: \ refresh=refresh, **kwargs)\n File \"/usr/lib/python3.6/site-packages/salt/modules/zypperpkg.py\"\
master: , line 2797, in resolve_capabilities\n refresh_db(root)\n File \"/usr/lib/python3.6/site-packages/salt/modules/zypperpkg.py\"\
master: , line 1300, in refresh_db\n out = __zypper__(root=root).refreshable.call('refresh',\
master: \ '--force')\n File \"/usr/lib/python3.6/site-packages/salt/modules/zypperpkg.py\"\
master: , line 351, in __call\n raise CommandExecutionError('Zypper command failure:\
master: \ {0}'.format(self.error_msg))\nsalt.exceptions.CommandExecutionError: Zypper command\
master: \ failure: Signature verification failed for file 'repomd.xml' from repository 'octopus-repo1'.\n\
master: Repository 'octopus-repo1' is invalid.\n[octopus-repo1|https://download.opensuse.org/repositories/filesystems:/ceph:/octopus:/upstream/openSUSE_Leap_15.2]\
master: \ Valid metadata not found at specified URL\nHistory:\n - Signature verification\
master: \ failed for repomd.xml\n - Can't provide /repodata/repomd.xml\n\nPlease check if\
master: \ the URIs defined for this repository are pointing to a valid repository.\nSkipping\
master: \ repository 'octopus-repo1' because of the above error.\nSome of the repositories\
master: \ have not been refreshed because of an error.\n"
master: duration: 14717.992
master: name: install required packages
master: result: false
master: start_time: '20:28:53.730077'
master: state: pkg_|-install required packages_|-install required packages_|-installed
master: [2020-04-30 18:29:08.913287] [node1.octopus-4n] Finished with failures
```

Start at the ceph-salt.software state named in the traceback, then read Salt's salt/modules/zypperpkg.py refresh_db and resolve_capabilities paths. Reproduce the zypper metadata or signature failure and define done as avoiding the traceback while reporting the repository refresh failure clearly; no test file is named.

Written by the indexing model from the issue text.