Only upgrade if remote server path differs from target of local symlink

[manics]

omego upgrade always does a restart even if the server version is the same. There should be an option to dereference the remote url and compare the basename of the remote server zip with the target of the OMERO.server symlink, and only upgrade if they're different.

[manics]

Also, would be nice if -n would report whether the upgrade would run or not, so that this could be used in ansible check-mode.