Simplify etckeeper profile
    
    On initial highstates, the etckeeper init state would fail due to
    git-core not yet being installed.
    Since git-core is listed in the common pillar and managed through the
    zypper formula, add a dependency to its installation state in the
    respective etckeeper ones.
    To streamline the setup, move the etckeeper packages to the common
    pillar as well, and to simplify it further, remove the obsolete
    SLE 12 conditional.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>