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
Use a tempfile.mkdtemp() based temporary directory

Change-Id: Ie0e8137615348b130b67323d0d2913dc5ebfd5fb
Fix up SAIO to use new gatekeeper

Fix up the SAIO default proxy-server pipeline to use the new
gatekeeper and update to follow the pipeline ordering from proxy
sample.

Change-Id: Iaa4de7abd7fc557cafe6fe40668973e9fc142ca3
Add debug logger on per-test proxy-server

To help debug problems with the proxy-server app setup for each class,
we add a debug logger so that on failure we can see what requests were
actually sent.

Additionally, we move the set_http_connect() call to the same relative
position as the one before it for clarity of intent.

Related bug 1271962

Change-Id: Idc301c06e114b11c358ee4fbc0b2b70ec7743091
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 "Fix up SAIO to use new gatekeeper"
Merge "Add debug logger on per-test proxy-server"
Merge "Use a tempfile.mkdtemp() based temporary directory"
Merge "Add documentation for GateKeeper"
Merge "ismount should not raise exceptions"