functional tests are often failing due to timeouts in `wait_for`
functions. It forces us to trigger a "recheck". Let's make it
configurable instead. A top level timeout can be set
with `OPENSTACKSDK_FUNC_TEST_TIMEOUT` env and defaults to 300.
Func tests for each service might override this var
(i.e. for heat `OPENSTACKSDK_FUNC_TEST_TIMEOUT_ORCHESTRATE`).
OPENSTACKSDK_FUNC_TEST_TIMEOUT_LOAD_BALANCER is set to 600 (in tox)
to respect the value previously used in the code.
Change-Id: I7bdcd67e858a1f2c43cb131ebdf549da070fbb4a
84116d6125