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"
Revert "Use auth_token_info to initialize heat_keystoneclient sessions"

It looks like this change has revealed an unrelated race with locking
during delete. While this issue is investigated then this change
needs to be reverted to fix the gate.

Related-Bug: #1306029

This reverts commit 535dd4e77e2c439a8611f27e5e9fa821cc8d7589.

Change-Id: I06be473d7c6961e399c4fd6128f374d24b9b0510
Merge "Revert "Use auth_token_info to initialize heat_keystoneclient sessions""