Support local roles for pre/post steps

[oybed]

Currently, the openshift-applier supports using a galaxy requirements file to pull in pre/post step roles. This works great, but can be confusing if the roles are kept in the same repo as the inventory used to drive the openshift-applier run. In that case, it would be great if the openshift-applier could support utilizing local roles that exists in the local repo. I.e.: make the applier check locally first, if not found, use the galaxy roles.