Rename pkg.available_version to pkg.latest_version

See https://github.com/saltstack/salt/issues/4163#issuecomment-15211619

All refs have been updated, and each pkg provider now has
available_version as a function reference pointing to latest_version.

Sphinx docs also updated so that the available_version function
reference is hidden in the docs.
Merge pull request #4183 from terminalmage/latest_version

Rename pkg.available_version to pkg.latest_version