qa: improve _pmdaprometheus_wait_for_metric() in common.prometheus
qa: improve _pmdaprometheus_wait_for_metric() in common.prometheus

no need to wait for config files in _pmdaprometheus_wait_for_metric()
since we can just wait for any of the prometheus.control metrics or
a named metric to be sure the PMDA has started and the needed metrics
have been dynamically created.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp