Support post jobs by supporting rev checkout

Currently zuul-cloner does not support post jobs, as it does not know
what to checkout.  This adds the ability on a per project basis to
specify a revision to be checked out.  When specified zuul-cloner
will successfully check out the same repo as gerrit-git-prep.sh does
in post jobs.

Sample usage:
clonemap:
  - name: openstack/neutron
    dest: ./neu
  - name: openstack/requirements
    dest: ./reqs

export ZUUL_PROJECT="openstack/neutron"
export ZUUL_NEWREV="a2Fhc2Rma2FzZHNkZjhkYXM4OWZhc25pb2FzODkK"
export ZUUL_BRANCH="stable/liberty"

zuul-cloner -m map.yaml git://git.openstack.org $ZUUL_PROJECT \
openstack/requirements

This results with openstack/neutron checked out at rev a2Fhc2 and
openstack/requirements at 'heads/stable/liberty'

Change-Id: Ie9b03508a44f04adfbe2696cde136439ebffb9a6
5 files changed