Update FTS REST API permissions and add roles

http://developer.couchbase.com/documentation/server/current/rest-api/rest-fts.html

1. Some permissions have changed since they were documented; these need to be made accurate.
2. Permissions are not directly settable by end users, so add roles, which end users can set. These will need to be changed in Spock as some new roles are added, but this ticket just covers 4.6
3. Keep permissions in there, because that's what the user will see in the audit.log for example and they will be needed when user defined roles are introduced.