+ fix for support of multiple mgmt servers


mysql-test/r/ndb_alter_table.result:
  added test case for bug #5431
mysql-test/t/ndb_alter_table.test:
  added test case for bug #5431
ndb/src/common/mgmcommon/ConfigRetriever.cpp:
  fix to make multiple mgmt srvrs work
sql/ha_ndbcluster.cc:
  fixed and added test case for bug#5431