start: call lxc_find_gateway_addresses early

start: call lxc_find_gateway_addresses early

This restores the lxc.net.x.ipv4.gateway = auto and
lxc.net.x.ipv6.gateway = auto functionality.

When the child is created the parent and child have different views of
struct lxc_handler since - obviously - virtual memory is duplicated. So any
changes to done by the parent that the child should see need to be IPCed to it.
For any non-actual device creation stuff this does not make much sense. This
includes finding gateway addresses. Move it back prior to clone().

Fixes #3078

Signed-off-by: Thomas Parrott <thomas.parrott@canonical.com>
[christian.brauner@ubuntu.com: non-functional changes and update commit message]
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>