Document migration of messaging subsytem to messaging-activemq subsystem

New operations were added to provide easy way of migration configuration from EAP 6 to EAP 7. In case of migration messaging subsystem (in EAP 6, HornetQ) to messaging-activemq subsystem (in EAP 7, ActiveMQ Atemis) new operations were added to CLI:
migrate
describe-migration

migrate operation migrates messaging subsystem to messaging-activemq
describe-migration operation is dry run and will print the same output as migrate operation. It's recommended to run describe-migration first to check possible warnings.