Quote all `rubber_env.host` in Monit config template

I got an error when starting Monit
And fixed by quoting `rubber_env.host`
https://groups.google.com/forum/#!topic/rubber-ec2/e762wVWG-e4
So I think maybe the host in all Monit config files should be quoted

Similar to an old PR
https://github.com/rubber/rubber/pull/76