Add "environment" getter to DSL

 Customer runs into a problem where they're using the private API of an older version of r10k in their Puppetfile like this:

We use the puppet_environment variable to set the default branch. For example:

puppet_environment = File.basename(@librarian.basedir)

default_branch = case puppet_environment

when/^feature_/

'development'

else

puppet_environment.split('_').first

end

Then for our repos we use the default_branch parameter

:default_branch => default_branch

It's been suggested that we add an environment or environment_name to get this functionality back ("So it looks like they want the "basedir" so they can get the name of the current environment so they can have a particular default_branch when doing feature branches. I think it would be a valid workflow (feature branch specifics are hard because you want them one way in the PR, but different when they're merged). I'd be happy to add an environment or environment_name getter to the DSL if other's are okay with it." - Justin Stoller)

Outcomes

• Add back the basedir getter
• Add a deprecation warning for using basedir
• Add an environment getter
• Open a ticket for removing basedir in 4.0.0
• Update docs in the r10k repo