Add debug_logstash config flag
If debug_logstash is true, the logstash poller will log logstash
queries and responses at debug level. debug_logstash defaults to
False because logstash query responses are very spammy in logs.
Added --debug flag to scap poll-logstash.
Note that the logstash query performed by the AbstractSync canary
check is still logged since it happens with low frequency (usually
once per deployment, which is at least 10 minutes).