Add ftp banners

[Enhancement] Add OVAL for ftp_present_banner and ftp_log_transactions checks

- add OVAL for ftp_present_banner for RHEL 6 & 7 and Fedora 20
- add OVAL for ftp_log_transactions for RHEL 6 & 7 and Fedora 20
- Enable OVAL use in RHEL 6 & 7