Add ConsistencyGroupStatus enum field

Add ConsistencyGroupStatus enum field

This change adds a new enum and field, ConsistencyGroupStatus and
ConsistencyGroupStatusField, that will hold the constants for the
'status' field of the ConsistencyGroup object. This enum and field
are based off the base oslo.versionedobjects enum and field. This
also changes over the ConsistencyGroup object to use the new field
(and bumps the version so newer versions know to enforce
valid values). Finally, all uses of strings for comparison and
assignment to this field are changed over to use the constants
defined within the enum.

Change-Id: Icee2727f0a550541876c63b6e7340acf3de553c8
Implements:  bp cinder-object-fields