bcfg2: Add inheritance to bcfg2_repo.config

With this change, it's now possible to use:
 - etc/<conf>.ini (default value, has to exist)
 - etc/<conf>.ini.GXX_<group> (group specific override)
 - etc/<conf>.ini.H_<hostname> (host specific override)

Note that instead of simply overriding the file entirely, the settings
are merged on top of one another starting with the default, then a
sorted list of groups (priority and name) and finally any host specific
entry.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>