CBQE-1101: support for ep-engine postconditions

Adding new EPStatChecker class to support checking workloads against
stats that can be collected against typical 'cbstats all' command.

i.e)
python cbsystest.py run workload --create 100 --ops 100 --cluster default --postcondition "ep_warmup_thread = running"

also support manually specifying which postcondition_handler function to use, which is useful in the case that both the
bucket engine or ep-engine can possible return same stat:
python cbsystest.py run workload --create 100 --ops 100 --cluster default --postcondition "ep_warmup_thread = running, epengine_stat_checker

Change-Id: I016d01b8980a4fa583fe3c5348e1c2069301abf8
Reviewed-on: http://review.couchbase.org/25199
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>