Migration CLI framework

Refactored migration import script to support the following:

- Multiple (potential) import adapters.
- Source project name vs. Target project name.
- Cache of imported resources/stories to avoid duplicate imports.
- Pre-warming of tags and users to reduce points of failure during
import.

CLI Syntax:
storyboard-migrate --from-project zuul --to-project openstack-infra/zuul

The import script will complain if the project does not yet exist on
the StoryBoard side.

Change-Id: I3ae381af0323de57d46b55501448d1cd41689a54