Add support for a 2+1 EC policy to be optionally used as default
policy when running in process functional tests.
The EC policy may be selected by setting the env var:
SWIFT_TEST_IN_PROCESS_CONF_LOADER=ec tox
when running .functests, or by using the new tox test env:
tox -e func-ec
Change-Id: I02e3553a74a024efdab91dcd609ac1cf4e4f3208