Enable custom BUILD_TIMEOUT

The BUILD_TIMEOUT is hardcoded in lib/tempest. This patch
enables people to pass a custom BUILD_TIMEOUT parameter

Change-Id: I260c32e7ed6bf9a49a6bc40272a5535afbca2731
Merge "Enable custom BUILD_TIMEOUT"
Merge branch 'master' of https://github.com/openstack-dev/devstack