(PUP-2143) Implement :enableable for OpenBSD service provider

Before now, Puppet has been unable to enable services on OpenBSD.

This work implements a flags property that is manages the content of the
rc.conf.local file, used by OpenBSD to define service runtime options
and enable services.

The flags property may also be used by other platforms, such as FreeBSD.
Merge branch 'pull-2495'