`omero db` should auto-detect serverdir if possible

[manics]

See #74 (comment) - there is a necessity postulated to use the --serverdir flag to point to the server in order to find the config (note that the config is needed as by default the config file is used to extract the info about the db). The symlink to the server which the flag is expecting could be by default being searched for, thus sparing the user a level of complication
(reported by @pwalczysko)