(maint) Adds acceptance tests into el6 configuration

Copies acceptance/tests which do not use with_master_running_on into
config/el6/tests.  This set of tests should run in the new testing with
packages Jenkins jobs.
[maint] Add missing helper libs
[maint] Add missing fixtures directory to el6/external_ca_support
[maint] Remove el6.tar.gz before recreating it

If the tarball exists and tar recreates it, and if the new tarball
size is smaller, tar succeeds but reports:
    tar: el6.tar.gz: File shrank by 93002 bytes; padding with zeros
and exits with a '1' which causes the job to report failure.

Removing the file first ensures this false positive won't happen.
Merge pull request #1744 from 'kylog/feature/testing-against-packages'

* kylog/feature/testing-against-packages:
  [maint] Remove el6.tar.gz before recreating it
  [maint] Add missing fixtures directory to el6/external_ca_support
  [maint] Add missing helper libs
  (maint) Adds acceptance tests into el6 configuration

Closes #1744