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, 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.