Introduce RBOVIRT_REST_TIMEOUT for tests

It looks like my testing boxes are sometimes slower and tests can take some
time. Introcued RBOVIRT_REST_TIMEOUT environment variable which overrides the
default 30 secods REST HTTP timeout to the oVirt API.
Can we just skip assigning a value to `:timeout` if the environment variable doesn't have a value? That way if the server changes the default timeout it will be used by default, instead of using always 30.