restapi: Show correct step type for async tasks

Earlier the step type details for gluster async tasks was being shown as
UNKNOWN for steps like REBALANCING_VOLUME and REMOVE_BRICKS.
Corrected the same in StepMapper to correctly map to valid step type.

Change-Id: I3cec46a382d05d9294bf7cca562167fd634ef63c
Bug-Url: https://bugzilla.redhat.com/1037709
Signed-off-by: Shubhendu Tripathi <shtripat@redhat.com>