eventlogging: couple less tightly to ganglia

1) Segregate code that depends on Ganglia in eventlogging::monitoring.
2) Make eventlogging::monitoring optional.
3) Don't include if hiera variable has_ganglia is set to false

Change-Id: I9eaffef2c0176d6860ba0709d2eaa6cdfedd8152
Merge "eventlogging: couple less tightly to ganglia" into production