Add file.list_backups execution function

This commit adds a function that will list the available backups for a
given path.
Add file.restore_backup, file.delete_backup

These functions allow you to restore a backup found with
file.list_backups, as well as remove unwanted backups.
add support for a2ensite,a2dissite
Document backup listing/restoring/deleting
make Mike happy
Detection and enumeration of GPUs works on a Mac now
switch command to __salt__['cmd.run']
add information to backup restoration docs
Merge pull request #6377 from KB1JWQ/osxfixgpu

Detection and enumeration of GPUs works on a Mac now
Merge pull request #6375 from doublerr/apacheTasks

add support for a2ensite,a2dissite
Merge pull request #6376 from terminalmage/issue6322

Add functions to manage file backups
Merge branch 'develop' into with-pylint-pragmas