Move Group Replication plugin from ./rapid to ./plugin folder.

Changes:
 rapid/plugin/group_replication -> plugin/group_replication
 rapid/unittest/gunit/group_replication -> unittest/gunit/group_replication
 rapid/unittest/gunit/libmysqlgcs -> unittest/gunit/libmysqlgcs
 rapid/plugin/group_replication/tests/mtr -> mysql-test/suite/group_replication
 rapid/plugin/group_replication/tests/mtr/inc -> mysql-test/include
 rapid/plugin/group_replication/tests/collections -> mysql-test/collections