When kolla-build is running, if there are network issues or if the
source's location (like http://tarballs.openstack.org) fails to
respond due to high number of concurrent requests, kolla-build just
hangs/blocks indefinitely.

This patch set resolves this issue by adding a timeout of 120
seconds for requests.get() in kolla/cmd/build.py, adds a unit test
for it in kolla/tests/test_build.py and also the "timeout" argument
in kolla/common/config.py.

Change-Id: I7c8745a20b9bd1c3f5d6a55c72a794f16fd7e513
Closes-Bug: #1548614