config: Fix RA interface config being overwritten

In case multiple dhcp uci sections are configured and one section contains
RA config while the other section does not; the RA interface config of the
first section will be overwritten by the second section.
Fix this by setting the default values for ra_management and ndp_proxy_routing
uci parameters when the interface is created and by removing the overwrite check

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>