webadmin: relax disk profile validation

Allow not passing the engine profile (to support lower
compatibility versions)

Change-Id: Ib4c9489659900f6855492c936bf71a6e2cb3c89a
Bug-Url: https://bugzilla.redhat.com/1085049
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>