Manual trigger of sync

Expose endpoint for triggering sync. Operator should be able to execute sync manually when he notices differences between Marathon and Consul states to resolve the issue faster.

Differences may occur e.g. when marathon-consul doesn't receive events from Marathon.

Things to consider:
- usability: when running multiple instances of marathon-consul as a cluster how to know which instance to call? Currently sync may be executed on the leader only.
- safety: would it be safe if manual sync and scheduled sync were running simultaneously? If not, how to introduce mutual exclusion?