couchbase-cli server-add 5.1.1 fails when executed against 4.1.X cluster

Problem
Stack trace when adding a 5.1.1 node to a 4.1.2 cluster using couchbase-cli on the 5.1.1 node:

Traceback (most recent call last):

  File "/opt/couchbase/lib/python/couchbase-cli", line 20, in <module>

    main()

  File "/opt/couchbase/lib/python/couchbase-cli", line 15, in main

    cli.execute(args)

  File "/opt/couchbase/lib/python/cbmgr.py", line 357, in execute

    opts.klass().execute(opts)

  File "/opt/couchbase/lib/python/cbmgr.py", line 1558, in execute

    if opts.storage_mode is None and settings['storageMode'] == "" and "index" in opts.services:

KeyError: 'storageMode'

Steps to Reproduce

1. Setup a 4.1.2 cluster and 5.1.2 node
2. Using Couchbase-cli on the 5.1.2 node to add the new node to the cluster

   /opt/couchbase/bin/couchbase-cli server-add -c 10.111.112.101:8091 -u Administrator -p password --server-add 10.111.176.104:8091 --server-add-username=Administrator --server-add-password=password --services data,index,query

Notes
While backwards is not supported we should do out best to try and maintain it where possible