Clean up SELinux handling in acceptance testing.

[ekohl]

This attempts to unify SELinux handling in the tests. It moves the package installation to the acceptance spec helper to reduce duplication. It then makes the set_apache_defaults line idempotent and restorecon_apache correctly chained. This works around PUP-10548 which is that Puppet doesn't reload file contexts within a run. That means it must first create the file(s) and then run restorecon to get correct contexts.