[CE] Error should show correct information in initial cluster with --index-storage-setting

Problem

When couchbase-cli us used again a CE to initial the cluster with the incorrect --index-storage-setting setting. The error produced says to use forestdb instead. However the couchbase-cli does not accept forestdb as a option.
The right option is default, the error message should make this clear as this cause confusion for the user.

Steps to reproduce

1. Install Couchbase Server CE 5.5.0-2907 on a centos 7.4 server
2. Using couchbase-cli to initialize this node with --index-storage-setting set to memopt which will fail as expected:

   [root@s44015 ~]# /opt/couchbase/bin/couchbase-cli cluster-init -c 127.0.0.1 --cluster-username Administrator --cluster-password password --services fts,data,query,index --index-storage-setting memopt

   ERROR: storageMode - The value must be one of the following: [forestdb]

3. Try agin using forestdb it fails again unexpectedly

   [root@s44015 ~]# /opt/couchbase/bin/couchbase-cli cluster-init -c 127.0.0.1 --cluster-username Administrator --cluster-password password --services fts,data,query,index --index-storage-setting forestdb