qa/workunits/mon: ensure election strategy is "connectivity" for stretch mode

qa/workunits/mon: ensure election strategy is "connectivity" for stretch mode

The election strategy is randomly chosen for this type of test. Sometimes,
the test passes if the "connectivity" election strategy happens to be picked.
But if a different strategy, i.e. "classic", is picked, then the test will fail.

We can ensure that the election strategy is "connectivity" by setting it in the
workunit with the ceph CLI command. Although connectivity was specified in
stretch-mode-5-mons-8-osds.yaml, that config ultimately gets overridden by
the "qa/mon_config" yaml.

Fixes: https://tracker.ceph.com/issues/69107
Signed-off-by: Laura Flores <lflores@ibm.com>