[Server-Manager: Update rabbitmq-config and correct report status for compute]

Closes-Bug:1399479

Issue:
1)Rabbitmq config was not update.
2)Mnesia database was deleted even before rabbitmq was stopped.
This might result in stale data ending up in the mnesia.
3)Compute status was not update properly.

Fix:
1)Update the config.
2)Move the delete of mnesia after the stop of rabbitmq-server.
3)Move the report_status within the block for Ubuntu and centos.

Testing:
1)Rabbitmq cluster was consistently formed.
2)Compute status was update to completed only after compute was able to
connect to vnc-server.

Change-Id: I34f3d46c5ef14f7489bae55d0d1f286370c2d819