This commit contains the following fixes:
* pass the apt config directory to the DpkgPM constructor, so one can
instantiate this class multiple times and give it different config
files (like for creating SDK);
* change constructor argument name from 'dpkg_archs' to 'base_archs';
* export APT_CONFIG environment variable before calling apt-get, not in
constructor. If done in constructor, the last class instantiation,
sets the environment, which is note desireable;
