Send swift upload instructions to workers

Have zuul send signed credentials as part of the job for workers to
consume and upload assets to a defined location.

Zuul currently doesn't care about logs however this change will
suggest a LOG_PATH to builders as a unqiue destination prefix
allowing zuul to know preemptively the destination.
The workers are still required to send a URL of the final location.

Change-Id: I042cdd2dd2407f381cafcabc5c6b83d9b9a9eb00
Merge "Send swift upload instructions to workers"
wmf: soften requirements

WebOb and paramiko have a minimal version which is enforced by OpenStack
but is most probably not needed.

ordereddict is for python 2.6, the feature is included in python 2.7.

Change-Id: I1aa1e3ae496d68d25dd64e8b21d2057c473342f4
wmf: make swiftclient an optional dependency

Swift is only used when there is a [swift] configuration section, but we
always import the swiftclient module.

I can not use the switclient on my setup, so I thought about loading it
via extras just like we do for statsd to transparently ignore the
missing dependency.

Another way would be only import it when we have a [swift] section but
that makes it harder to find out that is an optional dependency.

Change-Id: Ie2ef84f86f9f57796a85758147ba87d8b22c3e55