Because envoy didn't have permission for `envoy.log` file it would stop.
This made tests involving envoy fail.
Adding `ENVOY_UID` environment variable makes envoy run as root,
which avoids the problem.
Type: fix
Signed-off-by: Maros Ondrejicka <maros.ondrejicka@pantheon.tech>
Change-Id: I910416ad1c87137396e7da89c13de7739ce74c70