zebra: add all ipv6 global addresses to RA messages

zebra: add all ipv6 global addresses to RA messages

RFC 4861 states that ipv6 RA messages sent out an interface should
contain all global ipv6 addresses on that interface. This fix adds
that capability.  To override the default flags and timer settings
for a particular prefix, the existing "ipv6 nd prefix ..." command
should be used via vtysh under the appropriate interface.

Ticket: CM-20363
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>