Add import_exceptions to tox.ini

In oslo-incubator Oslo incubator multiple functions can be imported
from gettextutils on one line. So we should allow such imports in
hacking.

Change-Id: Ic55996987d312839c5e6863a1eb99faea3b575a9
Merge "Add import_exceptions to tox.ini"
Updated from global requirements

Change-Id: I4716bc17dd898ecf25f730641fc6901ade0da7eb
Log a cause of db backend connection failure

If opportunistic test case, i.e. fixture, can't connect to a test
database, it is happening silently, without any logs. Most of times we
need to know a cause of failure.

Change-Id: Ia9f93a2ed38d11ec22d7aaafc90e194b153c5393
Closes-Bug: #1306017