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.