MXS-1565: Invalid rejoin test

- 1 master, 3 slaves
- stop maxscale so it does not autorejoin later on
- stop & reset slave on servers 3 & 4
- add data to server 4
- restart maxscale, check that server 3 is rejoined but not server 4
- manually set server 1 to replicate from server 4, creating a relay master
- check that servers 2 & 3 are redirected, making server 1 just a slave
- switchover master to server 1, check that it's the master

Also, moved some common functions into their own files. These functions
are used by multiple tests.

maxscale-system-test/mysqlmon_failover_auto.cpp

@@ -3,7 +3,7 @@
  */
 
 #include "testconnections.h"
-#include "mysqlmon_failover_common.cpp"
+#include "failover_common.cpp"
 
 int main(int argc, char** argv)
 {