Add file.list_backups execution function

This commit adds a function that will list the available backups for a
given path.
a test...
undoing the test...
Backoff when minion fails to connect

NOTE: This is a potential change --- I haven't actually gotten a
test environment to try this out.

This adds a configuration option to the minion,
`acceptance_wait_time_max', that if set changes the acceptance
wait loop to add `acceptance_wait_time' seconds to the wait between
each iteration, up to this maximum value.

POTENTIAL FIX: Issue #5813
Better detection still for OS X
Remove accidentally added file
Merge pull request #6374 from michaelrice/develop

fix spelling error
Merge pull request #6373 from KB1JWQ/fixosvirt

Much better OS X virt detection now. It actually matches!
Merge pull request #6360 from kula/fix_5813

POTENTIAL FIX: Backoff when minion fails to connect
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
Populate __context__ with retcode; Fixes issue #5518
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