Support for config files

Support for config files

Initial support for reading service configuration from a file.
Currently, there is only one configuration setting, i.e.
'repo-cache-dir'. All configuration settings can be overridden with
environment variables.

By default, the service tries to read '/etc/obs/services/gbse' and
'~/.obs/gbs'. Other file(s) can be defined with the --config cmdline
option.

Also, change each test case to be run in individual temp dir and have
individual repo cache.

Change-Id: I8dbc713eaa06875f1f1b1163b3962f491dbce4e6
Signed-off-by: Markus Lehtonen <markus.lehtonen@linux.intel.com>