Updated RHEL7 XCCDF to reflect OS is RHEL 7
[rhel7] removing auxiliary/transition_notes.xml

This was made during RHEL6 STIG process. It may prove useful for RHEL7, but
dropping for now
Updated RHEL7 content to reflect RHEL 7

- Updated intro text
- Updated XSLT transforms to use RHEL7 in titles