When 'Depends-On: <change-id>' is used in a patch,
Zuul cloner must be used in order to ensure that
both patches are actually used for the gate test.
This patch implements the use of zuul-cloner [1]
for gate tests and falls back to using git if
zuul-cloner is not present.
[1] http://docs.openstack.org/infra/zuul/cloner.html
Change-Id: I2d5d7c94af7688d403b09e16565ef0086fa5c49c