Allow disabling of network activation (SC-307) (#1048)

In #919 (81299de), we refactored some of the code used to bring up
networks across distros. Previously, the call to bring up network
interfaces during 'init' stage unintentionally resulted in a no-op
such that network interfaces were NEVER brought up by cloud-init, even
if new network interfaces were found after crawling the metadata.

The code was altered to bring up these discovered network interfaces.
On ubuntu, this results in a 'netplan apply' call during 'init' stage
for any ubuntu-based distro on a datasource that has a NETWORK
dependency. On GCE, this additional 'netplan apply' conflicts with the
google-guest-agent service, resulting in an instance that can no
be connected to.

This commit adds a 'disable_network_activation' option that can be
enabled in /etc/cloud.cfg to disable the activation of network
interfaces in 'init' stage.

LP: #1938299

1 files changed, 11 insertions, 0 deletions

diff --git a/doc/rtd/topics/network-config.rst b/doc/rtd/topics/network-config.rst
index 8eb7a31b..494b687a 100644
--- a/doc/rtd/topics/network-config.rst
+++ b/doc/rtd/topics/network-config.rst
@@ -75,6 +75,17 @@ If `Cloud-init`_ 's networking config has not been disabled, and
 no other network information is found, then it will proceed
 to generate a fallback networking configuration.
 
+Disabling Network Activation
+----------------------------
+
+Some datasources may not be initialized until after network has been brought
+up. In this case, cloud-init will attempt to bring up the interfaces specified
+by the datasource metadata.
+
+This behavior can be disabled in the cloud-init configuration dictionary,
+merged from ``/etc/cloud/cloud.cfg`` and ``/etc/cloud/cloud.cfg.d/*``::
+
+  disable_network_activation: true
 
 Fallback Network Configuration
 ==============================