CI: update warden-cpi start script to force systemd

Based on this commit[1] we are mutating the in-container garden.ini
setting to make Warden use systemd instead of the garden-provide init
binary.

[1] cloudfoundry/bosh-warden-cpi-release@434738f#diff-f3d9c00d365d08274b8e73e1dc4fc4b2d38a92a654d4d2b27f4ffdc01730576bR1-R8

Author: aramprice

ci/dockerfiles/warden-cpi/start-bosh.sh

@@ -4,6 +4,10 @@ set -e
 
 local_bosh_dir="/tmp/local-bosh/director"
 
+# Update the on-container garden ini so that systemd is used as the INIT binary
+# See: https://github.com/cloudfoundry/bosh-warden-cpi-release/commit/434738fed168b71cc0c3ba8c038773cc1074189e#diff-f3d9c00d365d08274b8e73e1dc4fc4b2d38a92a654d4d2b27f4ffdc01730576bR1-R8
+sed -i 's/\/var\/vcap\/data\/garden\/bin\/init/\/sbin\/init/' /var/vcap/jobs/garden/config/config.ini
+
 /var/vcap/jobs/garden/bin/pre-start
 /var/vcap/jobs/garden/bin/garden_ctl start &
 /var/vcap/jobs/garden/bin/post-start