Initial input for HOT template guide and spec

This patch adds initial input for the HOT template guide and the HOT
specification.
Note that content so far is based on features implemented so far and
not forward-looking in any way. Whenever new features get implemented,
the guide and spec will be extended appropriately.

Contributes to blueprint hot-specification

Change-Id: I63d29ab0a1db6703b39f981161063d4fd9f97126
Enable multi-cloud standalone mode

Adds a multi_cloud configuration parameter for auth_password that
tells a standalone Heat installation to look for its auth_url in
the request headers instead of the configuration file.  Also adds
an allowed_auth_uris configuration option to specify which
auth_urls are allowed as targets.

bp heat-multicloud

Change-Id: Ic0929586ac3b57c7f9f3335e0dbd5e14e6b56067
Don't raise ResourceFailure directly in Instance

It's meant to be a wrapper around other exceptions that's invoked by
top-level resource operations; don't use it as an all-purpose exception.

Change-Id: I295202bca603006a4d5f5d56110fc4298ddd9cec
Merge "Enable multi-cloud standalone mode"
Merge "Don't raise ResourceFailure directly in Instance"
Merge "Initial input for HOT template guide and spec"