MODULES-1478: Add a $purge_connectors parameter.

See MODULES-1478.

Basically the way that this module adds additional Connector elements seems very un-Puppet-like.

This PR adds a parameter called $purge_connectors which removes all existing connectors for a given protocol so that if you define a single HTTP connector on port 8140 then that's the only HTTP connector you will have after the module has been applied. I think this is what most users will expect and want.

I've set it to false by default so that it's not a breaking change.
The second sentence is a little confusing, maybe reword to:
For example, if you define an AJP connector using `tomcat::instance::connector` and `$purge_connectors` is set to `true`, existing AJP connectors will be removed.