There may be deployments that only have a volume service but no compute
service, possibly as an intermediate step during the initial setup.
Don't fail the "volume list" command when no compute client can be
initiated.
Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: Idef77bdc8753a456a4c431b0385318dff6581a65