(maint) Untab mock templates, use install for chroot setup on el7

Adding tabs to the mock template makes it easier to read, but breaks
python's parsing of the file. Additionally, mixing groups and packages
means that we need to use install instead of groupinstall for the el7
case.
Merge pull request #74 from haus/maint/master/untab_mock_use_install

(maint) Untab mock templates, use install for chroot setup on el7