Implement muting monit securely

Goal: Have `monit unmonitor` and `monit monitor` events not sending alert mails

Idea: `monit-silent <action>` - executes `monit-alert none && monit <action> && monit-alert default` or similar

As discussed in https://github.com/cargomedia/puppet-packages/pull/648, https://github.com/cargomedia/puppet-packages/pull/554#issuecomment-47324762 and requested in https://bitbucket.org/tildeslash/monit/issue/66/add-alert-silencing-option