output-json-alert: conditionaly output metadata

output-json-alert: conditionaly output metadata

Metadata of the signature can now conditionaly put in the alert
events. This will allow user to get more context about the events
generated by the alert.

detect-metadata: conditional parsing

Only parses metadata if an output module will use the information.
Patch also adds a unittest to check metadata is not parsed if not
asked to.

output-json-alert: optional output keys as array

Update rule metadata configuration to have an option to output
value as array. Also adds an option to log only a series of keys
as array. This is useful in the case of some ruleset where from
instance the `tag` key is used multiple time.

(Jason Ish) rule metadata: always log as lists

After review of rule metadata, we can't make assumptions
on what should be a list or not. So log everything as a list.