Make user.info return {} if user not present

This change has been made in all four user providers.
Update refs to user.info

In 266b86a, user.info was changed in all the user providers to return an
empty dict if the user passed to it was not present. This commit updates
the refs in salt.modules.ssh so that they use dict.get (or check if the
return data from user.info resolves to False) where appropriate.

There are a few remaining refs in salt.states.user.present, but in all
of those remaining cases the user.info function is only run in instances
where the user already exists.
Merge pull request #4386 from terminalmage/issue4383

Make user.info return an empty dict when user not present