systemd: extend CapabilityBoundingSet for auth_pam

systemd: extend CapabilityBoundingSet for auth_pam

Auth_pam will require audit writes or the connection will be rejected
as the plugin fails to initialize like:
  openvpn[1111]: sudo: unable to send audit message
  openvpn[1111]: sudo: pam_open_session: System error
  openvpn[1111]: sudo: policy plugin failed session initialization

See links from https://community.openvpn.net/openvpn/ticket/918 for
more.

auth_pam is a common use case and capabilties for it should be allowed
by the .service file.

Fixes: #918
Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20180829142715.417-2-christian.ehrhardt@canonical.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg17432.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit a564781cfd9912d0f755394d1fa610706d93e707)