[CBBS/CBM] Don't pass password to cbbackupmgr

What is the problem?
In the backup service we always pass the username given to us by ns_server and the ns_server password of the current node obtained using cbauth to cbbackupmgr. cbbackupmgr doesn't actually use this password unless connecting to the backup service over gRPC fails.

This password isn't going to be correct for services other than ns_server on the current node, so falling back to it is incorrect.

What is the solution?
On the CBM side we don't need to run cbbackupmgr with the real password. On the cbbackupmgr side we should just error out if we have been started by the backup service but we cannot connect to it