[multi-site-vim] Document fernet key distribution for load balancer enabled tacker service

[Sripriya]

Currently tacker server node stores fernet keys for vim password decryption on local root file system. If Tacker service serves API requests through a load balancer, then the operation will fail if the request is not fulfilled by the server node which created and stored the fernet key.

This needs to be documented with a possible solution for syncing the keys across multiple server nodes