qa: mon/pool_ops.sh: fix pool tests

The '! command' doesn't fail properly, even with -e, in bash (wtf!).

Also, the last pool deletion command succeeds because the pool
'--yes-i-really-really-mean-it' doesn't exist.  So drop that test.

Signed-off-by: Sage Weil <sage@inktank.com>
qa: mon/pool_ops.sh: test pool set size

Signed-off-by: Sage Weil <sage@inktank.com>