[CBM] Behavior of the '-p' flag is odd/undefined when passwords start with '-'

What's the issue?
When supplying a password which is prefixed with -, the --password flag will attempt to prompt for a user password.

$ cbbackupmgr backup -a ~/Projects/couchbase-archive -r repo -c 172.20.1.1:8091 -u Administrator -p '-asdasd'

Password for -p:

$ cbbackupmgr backup -a ~/Projects/couchbase-archive -r repo -c 172.20.1.1:8091 -u Administrator -p -- '-asdasd'

Password for -p:

What's the fix?
We should correctly handle this case, or change the behavior of --password.

Is there a workaround?
Yes, use the CB_PASSWORD environment variable to supply the password instead.