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/.gitignore

@@ -167,6 +167,7 @@ mxs957
 mysqlmon_failover_auto
 mysqlmon_failover_manual
 mysqlmon_rejoin_good
+mysqlmon_rejoin_bad
 namedserverfilter
 no_password
 non_native_setup