REGTESTS: server: close an occasional race on dynamic_server_ssl.vtc

REGTESTS: server: close an occasional race on dynamic_server_ssl.vtc

Sometimes when sending commands to shut down a server, haproxy complains
that some connections remain, this is because the server-side connection
might not always be completely released at the moment the client leaves
and the operation is emitted. While shutting down server sessions work,
it seems cleaner to just use "option httpclose" which releases the server
earlier and avoids the race.

This can be backported to 2.5.