Refactor context module to make it more flexible

- context data is stored in a dictionary now, so any key is allowed;
- list of allowed key for certain context is stored in special class
  variable;
- methods "current" and "elevated" of ClimateContext class are
  accessible from module level;
- moved to usage of threading.local instead of emulating one in code (as
  long as we use eventlet's monkey-patching, it's specific local() will
  be used).

Change-Id: I3c79c9cd0d16dfb5c61208f2aa71b5c59f3f40f3
Merge "Refactor context module to make it more flexible"