teuthology-suite new feature: --newest [n]

[Dan Mick <dmick@redhat.com>]

If you don't use teuthology-suite, you can stop reading now.

I've added a new option to teuthology-suite, --newest, that will modify
how searching for builds works.

Currently, you can specify -c/--ceph to name a Ceph branch or -S/--sha1
to name a specific commit.  This chooses the version of Ceph that is
used to search for completed builds for the arch/distro/versions in your
run. The branchname is translated to a sha1 by looking up the branch on
a git server, and then the process is the same for both -c and -S: look
for that sha1 to have successfully built on all gitbuilders; if it
doesn't exist, the run fails.

With --newest, rather than failing, suite will try 'backtracking' along
the commit history of the branch/sha1, and searching for older commits
that have been built, backtracking as many times as you allow (by
default, 10 commits).  So, for instance, if you specify "-c master
--newest", suite will search for a build corresponding to the current
HEAD of master, as usual, but, if not found, will then try HEAD^, and
HEAD^^, and so on up to HEAD~10.  The newest commit that has a clean
build will be used for the run, and you'll see a log message about which
sha1 was chosen and how many commits older it was.

I expect this to be most-useful for nightly runs, but others may find it
useful as well.  Please try it if you wish, and send questions, bugs,
comments etc. to me or to the sepia list.

-- 
Dan Mick
Red Hat, Inc.
Ceph docs: http://ceph.com/docs