As suggested in feedback on
https://github.com/ansible/ansible/pull/17575, add
os_family to test_distribution_version. Add the
correct os_family to the existing testcase data
entries.
Also add os_family to the output of
gen_distribution_version_testcase.py so any new
generated entries will contain this data.
* add tests for centos6, rhel6 and rhel7
* gen_distribution_version_testcase with python2.6
* remove unused imports
* fix redhat/vmware/... parsing
* add centos7 test case