Set http_proxy to use local rack squid caches

We are currently running a Squid proxy on 192.168.1.100 in both Racks
This change may eventually belong in openstack-infra/config but
we can test and use it here for now...

Change-Id: Ic0aef792a44e473d2b75ad0091420ec1e1c2f4b9
Merge "Set http_proxy to use local rack squid caches"