saltstack network module fails with ecmp routes

Description of Issue

Salt network module not working when default route has several destinations.

Setup

Routing table looks like:

default proto bird metric 32 
	nexthop via 1.2.3.4 dev eth1 weight 1 
	nexthop via 2.3.4.5 dev eth2 weight 1 

and salt-call network.routes fails with:

[ERROR   ] An un-handled exception was caught by salt's global exception handler:
ValueError: u'nexthop' does not appear to be an IPv4 or IPv6 network
Traceback (most recent call last):
  File "/usr/bin/salt-call", line 11, in <module>
    salt_call()
  File "/usr/lib/python2.7/site-packages/salt/scripts.py", line 431, in salt_call
    client.run()
  File "/usr/lib/python2.7/site-packages/salt/cli/call.py", line 57, in run
    caller.run()
  File "/usr/lib/python2.7/site-packages/salt/cli/caller.py", line 138, in run
    ret = self.call()
  File "/usr/lib/python2.7/site-packages/salt/cli/caller.py", line 237, in call
    ret['return'] = self.minion.executors[fname](self.opts, data, func, args, kwargs)
  File "/usr/lib/python2.7/site-packages/salt/executors/direct_call.py", line 12, in execute
    return func(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/salt/modules/network.py", line 1686, in default_route
    _routes = routes()
  File "/usr/lib/python2.7/site-packages/salt/modules/network.py", line 1643, in routes
    routes_ = _ip_route_linux()
  File "/usr/lib/python2.7/site-packages/salt/modules/network.py", line 514, in _ip_route_linux
    address_mask = convert_cidr(comps[0])
  File "/usr/lib/python2.7/site-packages/salt/modules/network.py", line 1145, in convert_cidr
    cidr = calc_net(cidr)
  File "/usr/lib/python2.7/site-packages/salt/modules/network.py", line 1167, in calc_net
    return salt.utils.network.calc_net(ip_addr, netmask)
  File "/usr/lib/python2.7/site-packages/salt/utils/network.py", line 1087, in calc_net
    return six.text_type(ipaddress.ip_network(ipaddr, strict=False))
  File "/usr/lib/python2.7/site-packages/salt/ext/ipaddress.py", line 150, in ip_network
    address)
ValueError: u'nexthop' does not appear to be an IPv4 or IPv6 network

Versions Report

Salt Version:
           Salt: 2019.2.2
 
Dependency Versions:
           cffi: 1.6.0
       cherrypy: Not Installed
       dateutil: Not Installed
      docker-py: 1.10.6
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.7.2
        libgit2: 0.26.3
        libnacl: Not Installed
       M2Crypto: Not Installed
           Mako: 0.8.1
   msgpack-pure: Not Installed
 msgpack-python: 0.5.6
   mysql-python: Not Installed
      pycparser: 2.14
       pycrypto: 2.6.1
   pycryptodome: 3.7.3
         pygit2: 0.26.4
         Python: 2.7.5 (default, Aug  7 2019, 00:51:29)
   python-gnupg: Not Installed
         PyYAML: 3.11
          PyZMQ: 15.3.0
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.2.1
            ZMQ: 4.1.4
 
System Versions:
           dist: centos 7.7.1908 Core
         locale: UTF-8
        machine: x86_64
        release: 4.17.6-1.el7.elrepo.x86_64
         system: Linux
        version: CentOS Linux 7.7.1908 Core

Start in salt/modules/network.py at _ip_route_linux, where the traceback shows the failure while parsing routes, then inspect convert_cidr and salt/utils/network.py. Reproduce salt-call network.routes with the reported ECMP routing table. Done means the command handles multiple nexthop entries without raising the reported ValueError.