devstack: use GIT_DEPTH to reduce time required for git_clone
devstack: use GIT_DEPTH to reduce time required for git_clone

currently we clone entire of repos history, which is pretty large for
some of the large for some projects like nova/neutron. This history is
not very useful for CI and consume a lot time. It also generates
unnecessary load on the server.

By using GIT_DEPTH we restrict the depth of history of branch to 1
commit.

without depth:

time git clone https://git.openstack.org/openstack/nova.git --branch master
38.90s user
3.06s system
35% cpu
1:57.15 total

with depth:

git clone https://git.openstack.org/openstack/nova.git --branch master --depth 1
1.27s user
0.70s system
3% cpu
50.955 total

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