Extend vxlan_group option to allow a range of group addresses

Extend vxlan_group option to allow a range of group addresses

If vxlan_group is specified in CIDR notation, it is interpreted as a
range of group addresses. VXLAN VNIs are mapped to group addresses in
a many-to-one round robin fashion, or one-to-one if a large enough
range is provided. Since VNIs are 24 bits, a /8 such as 239.0.0.0/8
allows each VNI to use a unique multicast group. (239.0.0.0/8 also
happens to be the "site-local" multicast range.)

With multiple VNIs on a single multicast group, it is likely that
VTEPs will unnecessarily receive broadcast/unknown/multicast
datagrams for VNIs in which they do not participate. Using a range of
groups mitigates or eliminates this issue. It is thus an alternative
to the l2_population extension and driver for environments where both
multicast and linuxbridge are used.

The default setting is unchanged, but the comments in the ini file
suggest 239.0.0.0/8 as an alternative. Administrators are free to use
any valid multicast range that can be expressed in CIDR notation, and
should choose a size and starting address that make sense for their
environment.

DocImpact
Closes-Bug: #1477331
Change-Id: If9a3487a28ba2b02a6ef934c5421cec5d505b53c