Dell SC: Expanded comments and update var names

Dell SC: Expanded comments and update var names

Additional comments have been added to help explain what is happening
in the driver. This revolves mostly around dell_storagecenter_api.py
which is not an obvious file for a reviewer to review. Hopefully
additional comments will aid this.

Updated single character variable names except for r which is the
REST calls return throughout the code by convention.

Reworked create_volume for clarity. No functional changes.

Changed two obvious error messages from LOG.debug to LOG.error.

Driver version changed to 1.0.2.

Change-Id: Ib41d9bc45d20af4fc179b691549fe52f8bd6ec6d