Enable H302 check

H302 check was enabled in Horizon recently which allows importing modules only.
This patch fixes all import statements except for gettext where it makes more
sense to use _. In those cases the warnings are discarded by adding the # noqa
comment to the end of the line.

Fixes: bug #1218882

Change-Id: I745471bd372fac3724aa04cc08c512fb823cfbd0
Merge "Enable H302 check"