qa/suites/krbd: request msgr1 explicitly in unmap subsuite

qa/suites/krbd: request msgr1 explicitly in unmap subsuite

Since commit 5011cc926cd4 ("qa/suites/krbd: run unmap subsuite with
msgr1 only"), unmap.t is run only against msgr1.  pre-single-major.yaml
kernel has actually been gone for some time now, but there is still
value in maintaining a msgr1-only test.  With the default switched to
msgr2 in commit a577f6fa405c ("krbd: "rbd device map" command should
use msgr2 by default"), msgr1 needs to be requested explicitly.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>