restapi: Corrected input parameter types

The actions migrate, stopmigrate and activate on gluster bricks
collection accept the Action parameter. Corrected the same in RSDL.

Change-Id: I5405e80ca670957fd14e85aaa328e4716ff112a9
Bug-Url: https://bugzilla.redhat.com/1038678
Signed-off-by: Shubhendu Tripathi <shtripat@redhat.com>