Enable the usage of existing 'salt-call -m' option.

Currently, the ``-m`` flag for salt-call has no effect in the loaders.
This commit allows users to specify exactly the module dirs to load
given that they follow the naming conventions for 'grains', 'runners',
'modules', 'states', etc. This is a shortcut for having to refresh cache
state when using a masterless salt-call.
Merge pull request #3976 from dcolish/salt-call-use-cli-arg-modules

Enable the usage of existing 'salt-call -m' option.