Fix support for allow-hotplug statement in debian_ip network module

When an existing interface was configured with allow-hotplug or when setting
"hotplug: True" in network.managed state, this was not taken into account and
the allow-hotplug setting would be removed from /etc/network/interfaces.

This patch fixes this issue.
Merge pull request #13444 from bbinet/debian-network-hotplug

Fix support for allow-hotplug statement in debian_ip network module