Pass in GIT_BASE when calling stack.sh

since ./stack.sh is called as a binary, $GIT_BASE needs to be passed in
to override devstack's default GIT_BASE.

And add comment about git.openstack.org

Change-Id: I55482a387fe54235baba90e7b83837f7cb5b4bc6
Change default GIT_BASE to git.openstack.org

Instead of cloning from github, which ratelimits use our own
official git.openstack.org mirrors.

This change doesn't affect the gate as the images are pre-populated.

Change-Id: Ic12fbace0d54f7c624140410e2a2c1f5f8264d47
Add n-cond to list of nova processes to kill

n-cond was missing from the list of nova processes.

(plucked from gate)

Change-Id: I7aceff852de82b7e70294cf069c21d3399b27020
Merge "Pass in GIT_BASE when calling stack.sh"
Merge "Change default GIT_BASE to git.openstack.org"
Merge "Add n-cond to list of nova processes to kill"