Change osfinger to use os grain instead of osfullname

The osfinger is a concise mnemonic name representing OS and major
version. As such it should be derived from the concise, single-token,
OS name used by Salt. This also avoids trivial parsing issues
introduced when the osfullname contains spaces.

This patch replaces grains['osfullname'] with grains['os'] in the
creation of the osfinger grain.

NOTE: Exactly one os grain name contains spaces: 'Arch ARM'. (It
ought to be 'ArchARM', but that is not dealt with here.)