[Docs] Docs for TFTP server removal
[Docs] Docs for TFTP server removal

The tasks for this STIG are in an earlier patch, but the docs were
missing. This patch adds docs for TFTP server removal.

Implements: blueprint security-rhel7-stig
Change-Id: I13f8e29d417328131c3a25dbf150950b23258225

major/openstack-ansible-security,major/openstack-ansible-security