ismount should not raise exceptions

The changes from using os.path.ismount to using
swift.common.utils.ismount has caused problems since the new one
raises exceptions in cases where the old one did not. Daemons have
been encountering this and exiting; servers have been 500ing instead
of 507ing in this case, changing handoff behaviors, etc.

Since the new one was specifically written and tested for this new
behavior, I left that original function as ismount_raw and made
ismount do what it did before.

If there really isn't some reason for this new behavior, I'll be glad
to get rid of ismount_raw and just keep ismount. I couldn't see any
reason for the new behavior myself.

Change-Id: I2b5b17f9ed9656cd8804a5ed568170697d0b183d
Add documentation for GateKeeper

The purpose of GateKeeper mostly relates to the development of new swift code,
so I threw together a guide for development_middleware that covers some basics
with a eye towards metadata handling in-particular.

I also fixed up some missing autodoc's, split out middleware autodoc and added
some ref's here and about so I could link to them from the
development_middleware guide.

DocImpact
Change-Id: I20dd942ea8df9e33c3e794cb49669ffa1332c63e
Merge "Add documentation for GateKeeper"
Merge "ismount should not raise exceptions"