Added two more columns (deployment tag only for admins and number of

Added two more columns (deployment tag only for admins and number of
slices in memory). I also changed the source of some basic columns
to be retrieved on the fly by Comon. But if the source can be overriden
if there are also local tags with the same info.

I also changed the text motivating users to add slice description
at slice_add.php to match the one at slice.php