Added --rosdistro option, Rosdistro.get_target_arches(), and a bit of cleaning

The --rosdistro option will be used (in a later commit) to specify (if needed)
an http path to a rosdistro repository (containing releases/targets.yaml file, for example)
This will help creating local building farms

Rosdistro.get_target_arches(distro) is used to retrieve the arches associated to
a debian distro. This info will be stored in rosdistro/releases/arches.yaml with
the following structure:

groovy:
  precise: &pc_arm ['amd64', 'i386', 'armel']
  quantal: *pc_arm
  wheezy: ['armhf']