config: read the right config if run from subdir

A step towards being able to run GBP tools from subdirectories.

Now expands '%(top_dir)s' and '%(git_dir)s' in config file path to root
of the working directory and git metadata directory, respectively.

Also, adds a new method _read_config_file() in preparation for
supporting per-tree config files.

Fixes tests.test_Config: currently the only correct way to define the
config file(s) to be parsed is by using the GBP_CONF_FILES environment
variable.

Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>
Conflicts:
	gbp/config.py