shill: rtnl: Handle setting of peer address

Bonus changes: Rename "subnet_cidr" to "subnet_prefix".  Set a default
subnet prefix for OpenVPNDriver IPConfigs.

BUG=chromium-os:28003
TEST=Unit tests.

Change-Id: I953748b3ecb1a45237cedc18482c647eebe27efa
Reviewed-on: https://gerrit.chromium.org/gerrit/18461
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
diff --git a/dhcp_config.cc b/dhcp_config.cc
index ec74ff4..2511e2f 100644
--- a/dhcp_config.cc
+++ b/dhcp_config.cc
@@ -245,7 +245,7 @@
         return false;
       }
     } else if (key == kConfigurationKeySubnetCIDR) {
-      properties->subnet_cidr = value.reader().get_byte();
+      properties->subnet_prefix = value.reader().get_byte();
     } else if (key == kConfigurationKeyBroadcastAddress) {
       properties->broadcast_address =
           GetIPv4AddressString(value.reader().get_uint32());