Added build event and deploy to hooks

[robertstettner]

I have my server and main agent on my development AWS account, and have an agent on my production AWS account, and would like it to run pipelines that are deploy events and are deployed to production. In order to do that, I need to be able to use the DRONE_FILTER, on the agent setup with the event and deploy_to keys set to certain values.

Below is an example with labels that doesn't work because they don't support interpolation (it also looks hackish):

labels:
  - event=${DRONE_EVENT}
  - deploy_to=${DRONE_DEPLOY_TO}

And the agent having: DRONE_FILTER="event == 'deployment' and deploy_to == 'production'"

[CLAassistant]

All committers have signed the CLA.

[robertstettner]

I have patched my server with this to test it. All I can say is, that it works!

[vtolstov]

@tboerger @bradrydzewski ?

[bradrydzewski]

This code won't be merged because we are removing the mq package with a more stable replacement. Note that this was mentioned as a potential risk in gitter when we originally discussed the change [1]

The change will therefore need to be ported to #1954 and a strategy should be put in place for filtering on many fields, and not just deploy_to

[1] https://gitter.im/drone/drone?at=5894787c4c04e9a44e66a93a