[Enhancement] add OVAL check for multiple NTP servers

- Add check to shared directory and enabled for Fedora and RHEL 6 & 7
Update multiple NTP servers description
[RHEL/6] Add (RHEL-6) specific remediation for "Bind Mount /var/tmp To /tmp" rule
Merge pull request #332 from iankko/rhel6_mount_option_var_tmp_bind_add_remediation

[RHEL/6] Add (RHEL-6) specific remediation for "Bind Mount /var/tmp To /tmp" rule
Merge pull request #331 from redhatrises/ntpd_multiple_servers

[Enhancement] add OVAL check for multiple NTP servers