Changes

ansible: osmocom-obs-worker: add mount-securityfs.service

Fix errors in lxc-start related to not having securityfs available:

[   14s] booting lxc...
[   14s] lxc-start: obsbuild:root_6: lsm/apparmor.c: apparmor_prepare: 1132 If you really want to start this container, set
[   14s] lxc-start: obsbuild:root_6: lsm/apparmor.c: apparmor_prepare: 1133 lxc.apparmor.allow_incomplete = 1
[   14s] lxc-start: obsbuild:root_6: lsm/apparmor.c: apparmor_prepare: 1134 in your container configuration file
[   14s] lxc-start: obsbuild:root_6: start.c: lxc_init: 879 Failed to initialize LSM
[   14s] lxc-start: obsbuild:root_6: start.c: __lxc_start: 2008 Failed to initialize container "obsbuild:root_6"
[   14s] lxc-start: obsbuild:root_6: tools/lxc_start.c: main: 306 The container failed to start
[   14s] lxc-start: obsbuild:root_6: tools/lxc_start.c: main: 312 Additional information can be obtained by setting the --logfile and --logpriority options
[   14s] '/var/cache/obs/worker/root_6/.build/_exitcode' not found or symlink

Usually systemd would mount the securityfs automatically as I understand
it, but it doesn't work anymore without this service in our use case
where osmocom-obs-worker runs inside an LXC itself. This is probably
related to upgrading the opensuse version for the osmocom-obs-worker
containers.

Change-Id: I0c708815440c67340531548361deb326c864aefe