(PUP-1905) Split modulepath when supplied as an override from CLI

When a --modulepath option is supplied from the CLI to override the node
environment's modulepath, it can be made up of multiple directories with a
path separator.  The path is now split so multiple paths are correctly
searched.
Merge pull request #2426 from domcleal/tickets/master/pup-1905-multiple-modulepaths

(PUP-1905) Split modulepath when supplied as an override from CLI
Merge branch 'master' of https://github.com/puppetlabs/puppet