Disallow argument prefixing in couchbase-cli

By default argparse will recognize an argument prefix as long as it is unique. This allows users to specify only part of the long option. We want to disallow this and force typing the entire option so that we don't accidentally break scripts as new arguments are added. For example,

--cluster-fts-ramsize

and

--cluster-fts

would both be acceptable in the cluster-init command, but we only want --cluster-fts-ramsize to be accepted.