Revert "Support building wheels (PEP-427)"

'universal=True' means app supports py2 and py3 which isn't supported by
climate at least due to the eventlet dependency, so, my bad, it should
be reverted atm. W/o 'universal=True' wheel bdist still could be built.

This reverts commit 415a66430b3c395c8f442d89f4d4ec6d48378276

Change-Id: I074b33f1a1df87c3f3cc58b5734b6f20e364de5b
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"
Nova Aggregate wrapper to manage host allocation pools

Change-Id: Ife4949896c1679c941df5b7059a8fbcbf55ec287