Remove optional imports from clients.py

Why:
- These are in the requirements
- Heat is now well down the path of been a part of the integrated release
- All of the above clients are also integrated.
- If someone wants to use heat on an old OpenStack they still can
  as long as they install the required recent client libraries,
  which are backwards compatible with older OpenStack releases.

Change-Id: I39d7dcbd3e5655d3da1c97de5ff7eceb1826d4ea
Fixes-bug: #1174005
Merge "Remove optional imports from clients.py"