Add support for per-distro wheel mirrors

Because the wheel format does not do fine-grained enough selection on
things like the high likelihood that Ubuntu precise and Ubuntu raring
might have different underlying C libraries for python 2.7, we need to
put our built wheels into a structure that allows them to be used
safely.

lsb_release -r -i is used because Centos lists lsb_release -c as "Final"
which is not particularly helpful. This will wind up with identifying
strings like Ubuntu-12.04 and Centos-6.4 - which should be good enough
for what we need. The same logic is used in run-mirror to produce the
subdir.

Change-Id: I84c0ef57cf2e5e8c2cbfe2b18512b29228b0838f
Install requirements repo on slaves

In advance of starting to use the requirements project as the basis
for doing mirror selection, ensure that the repo is installed on all
of the jenkins slaves.

Change-Id: I893dd82e759f6a84a62400cdcbc7596b19ec5923
Merge "Add support for per-distro wheel mirrors"
Merge "Install requirements repo on slaves"