Do not rely on hash ordering in tests

With tox 1.7.2 hash randomization will be on by default, causing
random failures of our tests. How to reproduce:
* Update tox
* Run something like:
  $ for i in {1..20}; do tox -epy27 || break; done

Closes-bug: #1348818
(cherry picked from commit b056fca27897947ce5ef1326ee19db767f14f0ae)

Conflicts:
	ironic/tests/api/v1/test_drivers.py

NOTE(adam_g):  The following changes had to be tacked on here to get
through the stable/icehouse gate:

    * .testr.conf is also updated to exclude the in-tree Nova
       driver tests, which were never run until Juno and are
       breaking test runs for stable/icehouse.

    * tox.ini is updated to specify PYTHONHASHSEED=0 for doc builds,
      backported from 2abea080f617928af6386074e121aba33db2fe33.

Change-Id: I52c10c47a7a1c0e6a9e436efb090bbcb0fb02edc