net-test: packetdrill: support remote prefixes of arbitrary length

Allow the --remote_ip config parameter to be a prefix, not just a
single address. The entire remote prefix is routed to the tun device
or remote packetdrill machine.

The goal here is to all tests to use more than one remote IP. The
immediate motivation is testing GRE, where there is a remote IP for
the TCP endpoint and a separate remote IP for the GRE decapsulator.
But eventually we'll also want to allow testing multiple concurrent
TCP connections from multiple remote IPs addresses as well.

Here are examples of the remote prefix you get for various values of
the --remote_ip field. These are chosen for convenience and backwards
compatibility with existing tests that are already using the
--remote_ip parameter.

o Default:

configuration                      remote prefix
------------------------------     -----------------
IPv4, no --remote_ip            -> 192.0.2.0/24   (default)
v4/6, no --remote_ip            -> 192.0.2.0/24   (default)
IPv6, no --remote_ip            -> 2001:DB8::0/32 (default)

o Explicit IP without prefix length (prefix includes all bits):

configuration                      remote prefix
------------------------------     -----------------
IPv4 --remote_ip=10.248.0.3     -> 10.248.0.3/32
v4/6 --remote_ip=10.248.0.3     -> 10.248.0.3/32
IPv6 --remote_ip=2001:db8::3    -> 2001:db8::3/128

o Explicit with prefix length:

configuration                      remote prefix
------------------------------     -----------------
IPv4 --remote_ip=10.248.0.3/16  -> 10.248.0.0/16
v4/6 --remote_ip=10.248.0.3/16  -> 10.248.0.0/16
IPv6 --remote_ip=2001:db8::3/16 -> 2001::0/16

Change-Id: Ibc95c3419d66bcf1bbfd8a1fc7650d0003345ef2