gluster: enhance vdsmcli error handling

Vdsmcli also treated as a functional testing tool. Checking
the functional parameter could hide the state of depending
function. Engine might call those functions without these
parameters also. So the parameter null check should go inside
the gluster function.

Change-Id: I2e29dfb724cf4c8370bf836d64510376df37058a
Signed-off-by: Timothy Asir <tjeyasin@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23057
Reviewed-by: Bala.FA <barumuga@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>