Add `newrelic_enabled` variable

The default for `newrelic_enabled` is `True`, but it can be set to
`False` if for some reason the New Relic System Monitor Daemon should
not be provisioned on a host or a group of hosts, e.g. on development
virtual machines.
Update documentation to include `newrelic_enabled`

Mention presense of the variable and give an example
Merge pull request #1 from Recras/newrelic_enabled-var

`newrelic_enabled` var