ovsdb: Remove read permission of *.db from others.

ovsdb: Remove read permission of *.db from others.

Currently, when ovsdb *.db is created by ovsdb-tool it grants read
permission to others.  This may incur security concerns, for example,
IPsec Pre-shared keys are stored in ovs-vsitchd.conf.db.
This patch addresses the concerns by removing permission for others.

Reported-by: Antonin Bas <abas@vmware.com>
Acked-by: Mark Gray <mark.d.gray@redhat.com>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>