shill: vpn: Set the friendly VPN service name.

Also, implement a more concise KeyValueStore string lookup method and add unit
tests.

BUG=chromium-os:27775
TEST=unit tests

Change-Id: If6ab38a9110c09275816bcc6ca992a77425859eb
Reviewed-on: https://gerrit.chromium.org/gerrit/18106
Commit-Ready: Darin Petkov <petkov@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/key_value_store.cc b/key_value_store.cc
index faba479..26e2b3b 100644
--- a/key_value_store.cc
+++ b/key_value_store.cc
@@ -56,4 +56,9 @@
   uint_properties_[name] = value;
 }
 
+string KeyValueStore::LookupString(const string &name,
+                                   const string &default_value) const {
+  return ContainsString(name) ? GetString(name) : default_value;
+}
+
 }  // namespace shill