Recover environments that are stuck stopping

When jobs that stop environments fail, the environment can get stuck in
a state of "stopping." We want those environments to recover to a state
of "available."

This commit addresses that concern with the following:
- adds a new state event to environments to represent going from
  "stopping" back to "available"
- adds a new worker to fire the state event given the proper conditions
- makes deployables enqueue that new worker when they fail
- adds changes from worker related rake tasks related to adding a new
  worker
- adds specs
- updates environments spec factory

Changelog: fixed