Remove unnecessary sleeps

Every time MaxScale was restarted, the test waited for a total of 25
seconds. As MaxScale should start before the command returns, the sleep is
not necessary.

Added StartLimitBurst=0 to the systemd service file to allow rapid
restarts of MaxScale.

maxscale-system-test/testconnections.cpp

@@ -701,19 +701,15 @@ int TestConnections::close_maxscale_connections()
 
 int TestConnections::restart_maxscale()
 {
-    sleep(15);
     int res = ssh_maxscale(true, "service maxscale restart");
     fflush(stdout);
-    sleep(10);
     return res;
 }
 
 int TestConnections::start_maxscale()
 {
-    sleep(15);
     int res = ssh_maxscale(true, "service maxscale start");
     fflush(stdout);
-    sleep(10);
     return res;
 }