qa/cephfs: make code for generating health warnings reusable

qa/cephfs: make code for generating health warnings reusable

Code to generate MDS_TRIM and MDS_CACHE_OVERSIZED health warnings is
repeated in test methods of TestMDSFail and TestFSFail. Move this code
to separate helper methods so that it can be reused instead of
duplicating it. And move these helper methods to TestAdminCommands so
to make them conveniently available for reuse.

Signed-off-by: Rishabh Dave <ridave@redhat.com>
(cherry picked from commit 79b047b22e1031df74584cfa4b77d7b824b0e42e)