Allow alternate enc names to be used for ssh_auth states

If 'rsa' or 'dss', etc. are used as the 'enc' value in ssh_auth.present
states (rather than 'ssh-rsa' or 'ssh-dss'), the call to
salt.modules.ssh.check_key() will fail, which causes ssh_auth.present
states to mistakenly report the key as needing to be changed. This
commit adds a call to salt.modules.ssh._refine_enc(), which normalizes
the 'enc' param so that the call to ssh.check_key() does not fail.

Fixes #5374.