Pre/post tasks should be able to reference local roles

[jkupferer]

Currently openshift-applier always runs ansible-galaxy to pull roles for pre/post steps. This makes it difficult to test changes in pre/post roles, makes it difficult to run applier in secure environments, and makes it impossible to build an image with applier with baked-in tested version of ansible roles.

It should be possible to specify a local roles path for pre/post steps, perhaps with a design like:

    post_steps:
    - role: roles/openshift_machine_api
      local_role: true