Moves deployment trigger to separate rake task
Moves deployment trigger to separate rake task

This change facilitates additional flexibility when configuring the
deployment apps and environments.

In one case in particular, we have two apps deployed from the same
codebase with different sets of environments. By separating the
triggering from the container build task, we can call the trigger task
multiple times with different configuration.

Requires some adjustment to existing calls. A call like this:

rake deploy:docker DEPLOY_APPS=app1,app2 DEPLOY_ENVS=staging:staging,staging:preview

Would now be 2 calls:

rake deploy:docker
rake deploy:trigger DEPLOY_APPS=app1,app2 DEPLOY_ENVS=staging:staging,staging:preview

Or optionally 3:

rake deploy:docker
rake deploy:trigger DEPLOY_APPS=app1 DEPLOY_ENVS=staging:staging,staging:preview
rake deploy:trigger DEPLOY_APPS=app2 DEPLOY_ENVS=staging:staging

ad2games/docker-deploy,ad2games/docker-deploy,ad2games/docker-deploy