The RBD driver has supported discard (trim/unmap) since [1]. This patch
makes the driver advertise that it does so. Based on how the setting is
implemented, it is safe to hard code the value:
"Report to clients of Cinder that the backend supports discard
(aka. trim/unmap). This will not actually change the behavior of
the backend or the client directly, it will only notify that it
can be used."
[1] I094163b99d858384584966d65e9d8756c74a0c15
Change-Id: I2f1a4214c3a0561e6df99e8234a9b98f14139264
c3b96f88e8