Not assign dynamic IPv6 address on dhcp interface

Not assign dynamic IPv6 address on dhcp interface

When SLAAC IPv6 subnet using external router advertisement
(ipv6_ra_mode=None, ipv6_address_mode=slaac) is added to a
dual-stack network, the device of dnsmasq or router gateway may
already be assigned with dynamic SLAAC IPv6 address. Current
code only checks for permanent address on a device when
initilizing the l3 address of that device. This results
assigning duplicated address to dhcp tap device, which leads
to dhcp agent failure.

This fix changes dhcp code to not init dynamic IPv6 address
on dhcp device so dynamic address can be auto-configured by
either internal or external RA device.

Change-Id: I72b85eb93d1849299d523eedf563ab201b4a0352
Closes-Bug:1409634