Implement retype in IBM GPFS Driver and refactor

Implement retype in IBM GPFS Driver and refactor

1. Add retype method to GPFS driver.  Currently only storge_pool
   attribute is handled.  Future patches will extend this.
2. Refactor migrate method to handle migration locally if source
   and target are in the same gpfs cluster.
3. Move driver into 'ibm' directory.
4. Refactor based on pylint feedback.
5. Replace mox with mock in unit test.
6. Improve exception handling and logging.

DocImpact

Implements: blueprint gpfs-volume-retype
Change-Id: Ibc3bc367e5d42940c69d6747774c82c8cad22a35