Provide an option to omit "output" field in JSON output events.

[alukyan]

When Falco uses JSON format for output events the information in "output" and "output_fields" is duplicated. When Falco events are automatically processed by another system it is often not necessary to have human readable "output" part of the event, which almost doubles the size of the message leading to extra overhead in sending, processing and storing events externally. It is always possible to parse the message and remove unwanted part, but providing simple option to omit human readable part of the event right during event generation seems to be simplest solution.

{"output":"16:59:59.652105834: Informational user=root pid=93384 process=sh 
cmdline=sh -c iptables --version ppid=1813 parent=python3 
pcmdline=python3 -u bin/WALinuxAgent-2.2.21-py2.7.egg -run-exthandlers gparent=python3 ggparent=systemd gggparent=<NA> ggggparent=<NA>",
"priority":"Informational","rule":"New process started", "time":"2018-03-26T16:59:59.652105834Z",
"output_fields": {"evt.time":1522083599652105834,"proc.aname":null,"proc.cmdline":"sh -c iptables --version",
"proc.name":"sh","proc.pcmdline":"python3 -u bin/WALinuxAgent-2.2.21-py2.7.egg -run-exthandlers",
"proc.pid":93384,"proc.pname":"python3","proc.ppid":1813,"user.name":"root"}}