simple-init: add configurable RA timeout with network-manager

This is a follow-on to I475a253091cbaf63687b91c748c31a6753bb0f57 as we
are still seeing issues on some clouds with unconfigured networking.

We increase the timeout, but also make it configurable so we can
fiddle it without a dib release in the gate.

To follow-on from the experimentation done by clarkb, I can confirm by
emperical testing on a Centos 7 image (from today, today being this
change's date) that setting

 net.ipv6.conf.all.autoconf=0

by itself is "fatal" and the interfaces do not come up; i.e. nm does
not by default seem to re-enable ipv6 for the interface.  However,
explicitly adding:

 IPV6INIT=yes
 IPV6_AUTOCONF=yes

to the interface file *does* seem to make it work, even if
"all.autoconf=0" is set (then again, there's also bugs about the
effect of this [1]).  However, no extant distribution (I can currently
find) does anything like this by default.

If this continues, this may be an option.  Another might be to avoid
the use of the nm-settings-ifcfg-rh profiles and move directly to nm
ini files with glean.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=11655

Change-Id: I869ebffc8cde3bbff573f6583fd9dd02a5598590

diskimage_builder/elements/simple-init/post-install.d/80-simple-init

@@ -48,5 +48,6 @@ if [[ ${DIB_SIMPLE_INIT_NETWORKMANAGER} != 0 ]]; then
     # Debian has a many year old long bug report detailing this behavior with
     # the most interesting comment being at the end:
     # https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=755202#331
-    echo 'net.ipv6.conf.default.router_solicitation_delay=15' >> /etc/sysctl.conf
+    DIB_SIMPLE_INIT_NETWORKMANAGER_IPV6_DELAY=${DIB_SIMPLE_INIT_NETWORKMANAGER_IPV6_DELAY:-30}
+    echo "net.ipv6.conf.default.router_solicitation_delay=${DIB_SIMPLE_INIT_NETWORKMANAGER_IPV6_DELAY}" >> /etc/sysctl.conf
 fi

releasenotes/notes/simple-init-nm-delay-f579e05467785219.yaml

@@ -0,0 +1,11 @@
+---
+fixes:
+  - |
+    The ``DIB_SIMPLE_INIT_NETWORKMANAGER_IPV6_DELAY`` variable is
+    added for the ``simple-init`` element to set the
+    ``net.ipv6.conf.default.router_solicition_delay`` sysctl setting
+    (it defaults to 30s).  If you are seeing an extra "blank"
+    interface in ``nmcli con show`` it is likely that NM has not
+    correctly configured itself in the presence of an interface
+    already configured with IPV6 by the kernel.  Increasing this value
+    may help work-around the problem.