Stop security-manager before unmounting /opt/usr

Stop security-manager before unmounting /opt/usr

Security-manager creates mounts in a mount namespace that are invisible to
oded. Although /opt/usr is unmounted in oded's namespace it is still mounted
in the one used by SM. As a result device mapper can't use the device to
load the table.

This commit adds security-manager.service and socket to the list of known units
that have to be killed before unmounting /opt/usr. Socket is stopped to prevent
security-manager from being restarted. This is just a temporary solution. It
does not prevent other services from blocking ode by using mount namespaces.

Change-Id: I53584f17efc56fa39a503025d4f68010c3b3dbb3