REGTESTS: write a full reverse regtest

REGTESTS: write a full reverse regtest

This test instantiates two haproxy instances :
* first one uses a reverse server with two bind pub and priv
* second one uses a reverse bind to initiate connection to priv endpoint

On startup, only first haproxy instance is up. A client send a request
to pub endpoint and should receive a HTTP 503 as no connection are
available on the reverse server.

Second haproxy instance is started. A delay of 3 seconds is inserted to
wait for the connection between the two LBs. Then a client retry the
request and this time should receive a HTTP 200 reusing the bootstrapped
connection.