Now the test node sends a  `--frontend_url` parameter to the test suite, which an IPv4-enabled URL to the test node's backend. This is required to run remote tests on environments without IPv6 connectivity. 

Also the SlapOSController now waits 20 seconds instead of 10 seconds for the SlapOS proxy web server to start answering requests. On slower machines 10 seconds weren't being enough.

@Tyagov, please review.

/reviewed-on nexedi/erp5!189