Mask lxc-net.service for systemd

On systemd-based systems, lxc-net is started whenever
lxc is started.  This is because lxc-net is declared
as a dependency of lxc.

We currently mask the service on upstart systems.
We are presently manually configuring the lxcbr0
on all systems, systemd-based systems included.

This patch will mask the lxc-net.service to enable
lxcbr0 to be brought up without error due to conflicting
lxc-net.service.

Closes-Bug: #1580766
Change-Id: I099cf638bbb8874ca45b815383de8588fecbe7b6

tasks/lxc_net.yml

@@ -41,7 +41,7 @@
   tags:
     - lxc-net
 
-# All installations of LXC on init based systems (upstart primarily) will need to have the
+# All installations of LXC will need to have the
 #  the lxc-net service override in place to ensure its not restarted on system boot
 - name: Drop lxc-net override file for upstart
   template:
@@ -56,6 +56,17 @@
     - lxc-files
     - lxc-net
 
+- name: Mask lxc-net systemd service
+  file:
+    src: /dev/null
+    path: /etc/systemd/system/lxc-net.service
+    state: link
+  when:
+    - pid1_name == "systemd"
+  tags:
+    - lxc-files
+    - lxc-net
+
 # All Debian based systems use the interfaces.d directory for extra network configs
 #  this check ensures the needed source line is in the base config file
 - name: Ensure networking includes interfaces.d