Fix gate to use world writeable docker socket

A recent change related to sudo securepath broke how sudo -E
works.  Now the PATH is reset by sudo as set by tox.  As a result
we can no longer rely on sudo -E anywhere in our gating system
relating to path inheritence from the parent shell.

This patch uses a shell operation in the setup_nodes.yml code
to chmod the docker socket to 666 so docker containers can build
properly.  Now docker operations don't return a permission denied
and we no longer require the sudo -E operation in any of our tox
scripts.  This isn't a security vulnerability because our gate
scripts are only meant to be run in OpenStack infrastructure.
To make this more clear I recommend moving these shell scripts to
tests/gate.

Change-Id: I5b3d6e280e3c12a25defedb14b5589ba642043fa
Closes-Bug: #1562383