This patch adds the `memcached_disable_privatedevices` variable that
allows deployers to disable PrivateDevices in the systemd unit file.
This is a workaround to fix the systemd/LXC issues with bind
mounting an already bind mounted `/dev/ptmx` inside the LXC
container.
See Launchpad bug, lxc/lxc#1623, or systemd/systemd#6121 for more
details.
The is_metal variable is removed as it is unused.
Related-bug: 1697531
Change-Id: Id7c148bf901354a3dfc2f189ec659f2b92fc7985
a9acd22e82