s3:gencache: Allow to open gencache as read-only

s3:gencache: Allow to open gencache as read-only

This allows client tools to access the cache for ready-only operations
as a normal user.

Example:
    net ads status

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14370

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Fri May 15 14:40:32 UTC 2020 on sn-devel-184

(cherry picked from commit 04f0c45475de383a0be4ca355ab9aa7784e61c27)