Implement pluggability for tempest (exceptions)

First patch of three patches:
->1. exceptions
  2. config (+ module_utils.py)
  3. clients

The main intend is in moving currently existed single 'exceptions' module in
exceptions tree.
It allows separate exceptions by differencies:
- base
- main/common
- project-specific

It also allows to simplify search and using of exceptions, merge changes into
Tempest with minimized amount of merge conflicts.

Partially-implements: bp make-tempest-pluggable

Change-Id: I5c50affba0fce721583a6d56216e56c94ec3b188
Merge "Implement pluggability for tempest (exceptions)"