qa/tasks: do not cancel pending pg num changes on mimic

qa/tasks: do not cancel pending pg num changes on mimic

mimic does not support auto split/merge, but we do test mimic-x on
nautilus, which ends up with failures like:

ceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_py2/teuthology/contextutil.py", line 34, in nested
    yield vars
  File "/home/teuthworker/src/git.ceph.com_ceph_nautilus/qa/tasks/ceph.py", line 1928, in task
    ctx.managers[config['cluster']].stop_pg_num_changes()
  File "/home/teuthworker/src/git.ceph.com_ceph_nautilus/qa/tasks/ceph_manager.py", line 1806, in stop_pg_num_changes
    if pool['pg_num'] != pool['pg_num_target']:
KeyError: 'pg_num_target'

so we need to skip this if 'pg_num_target' is not in pg_pool_t::dump().

this change is not cherry-picked from master, as we don't test
mimic-x on master.

Signed-off-by: Kefu Chai <kchai@redhat.com>