RTD Allow projects to change their default-version

RTD Allow projects to change their default-version

Allow docs release via jjb config
Changes the default landing page from /latest/ to /stable/

by default we se to /latest/ (a no-op)
And on release change to stable via jjb config.

read the docs, by design creates /latest/ and /stable/ landing points
for the user.

If read the docs sees a tag or a new branch, it creates a /stable/
directory, which follows the latest tag or if there is no tag, the
latest branch.

Related issue where I ask for clarification as to their default
behavior in this regards.
https://github.com/readthedocs/readthedocs.org/issues/6415

ISSUE: RELENG-2549
Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>
Change-Id: I33a8df14e68340ba268c953a471daa02ee6fe766