Fix handling of lsb_release info in Debian

Debian's ``lsb_release.get_distro_information()`` returns a dict with keys
missing the leading 'DISTRIB_' found in /etc/lsb-release. This causes
the values to be assigned to grains like ``lsb_id``, ``lsb_release``,
``lsb_codename``, etc. instead of ``lsb_distrib_id``, ``lsb_distrib_release``,
``lsb_distrib_codename`` as the later grains code expects to find. As I am
not sure if this is a change in the return data for
``lsb_release.get_distro_information()``, this commit should preserve
backwards compatibility by only inserting 'distrib_' into the grain's
name if it was not already present.