spiderpig: Don't start logstash poller if logstash_host is blank

Make the logstash poller optional by not running it if logstash_host
is blank.

Set logstash_host to blank in local-dev/scap.cfg since there is no
logstash server that it can access. This avoids spam in the container
logs.

Tested using local-dev.

Edited by Ahmon Dancy

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