Enable H304 hacking check

The error was:

'import install_venv_common as install_venv' is a relative import

To fix it __init__.py was created and import string was fixed.

Requirements for hacking changed to >=0.5.6

Change-Id: I71aa7c7cb268c26aa00828306138a84b30289cb9
config.generator: replace sys.prefix by /usr

Nova has some default configuration option (bindir) value that uses
sys.prefix as path start. That can generates weird paths depending on
the system where you run the configuration generator.

This patch fixes that by hardcoding a replacement of sys.prefix to /usr,
which is what's actually expected most of the time.

Change-Id: Ied3b58695965a3102670e3761701fbe10ac74785
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "config.generator: replace sys.prefix by /usr"
Merge "Enable H304 hacking check"