CLI admin diagnostics: only check readability of var folder

In a multi-user environment where group members have permissions to read but
not write the var/ folder of the server, bin/omero admin diagnostics currently
fails with a "Cannot access" exception. This commit relaxes the check_access()
condition of the diagnostics subcommand to check the folder (and subfolders)
readability only.
Merge pull request #3115 from sbesson/diagnostics_readonly

CLI admin diagnostics: only check readability of var folder