RFR: Timer should not be sensor

## What? 

Timer is not a sensor anymore. It would now be part of st2 process (rules_engine for now but we can move if needed)

## Things fixed

* st2reactor/contrib doesn't exist anymore as webhook and timers are now moved.
* Registration of system trigger models now happen programatically (in webhook controller and st2timer) as opposed to bootstrap script based registration.
* Also with this PR, I refactored st2reactor/container/utils and moved trigger type creation method to st2common/services/triggers so it can be shared. 

## TODO
- [ ] https://github.com/StackStorm/st2/issues/913
Just want to confirm - the way timer is implemented, it doesn't block, right?

(I assume that internally it uses time.sleep and periodically checks for matching timers and this should be fine if we use eventlet monkeypatching).