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_unittest.cc b/dhcp_config_unittest.cc
index aa853b1..db6ca29 100644
--- a/dhcp_config_unittest.cc
+++ b/dhcp_config_unittest.cc
@@ -133,7 +133,7 @@
   IPConfig::Properties properties;
   ASSERT_TRUE(config_->ParseConfiguration(conf, &properties));
   EXPECT_EQ("4.3.2.1", properties.address);
-  EXPECT_EQ(16, properties.subnet_cidr);
+  EXPECT_EQ(16, properties.subnet_prefix);
   EXPECT_EQ("64.48.32.16", properties.broadcast_address);
   EXPECT_EQ("8.6.4.2", properties.gateway);
   ASSERT_EQ(2, properties.dns_servers.size());