Add --pub-key= and --identity to gscloud-server-create

`gscloud-server-create` creates a storage for you by default, --with-template allows you to select the corresponding template. The password is auto-generated for you.

Example:
```
$ gscloud server create \
--name centos8 \
--cores=1 --mem=1 \
--with-template="CentOS 8" \
--hostname centos8
```

Add three new flags to `gscloud-server-create`:

Each of those adds an SSH public key to the new root storage's authorized_keys file.

- `--pub-key=` takes a string argument as public SS key, uploads as new SSH key object, automatically derives a reasonable object name for it
- `--identity=` same thing, just accepts a path to a SSH key file or identity much as ssh(1)
- `ssk-key=` uses an existing SSH key object already uploaded to gridscale