security: Update Signature abstraction to match 0.1.1 spec

This commit removes intermediate SignatureWithPublicKey data
structure.  All uses of such data structure should be replaced with just
Signature and check that KeyLocator is present (hasKeyLocator()).

Change-Id: I1157e05c6a717ea28df91c4bfd76c7145b520a15
Refs: #1750
diff --git a/tests/unit-tests/security/test-signature-sha256-ecdsa.cpp b/tests/unit-tests/security/test-signature-sha256-ecdsa.cpp
index bcf01be..34f25bf 100644
--- a/tests/unit-tests/security/test-signature-sha256-ecdsa.cpp
+++ b/tests/unit-tests/security/test-signature-sha256-ecdsa.cpp
@@ -57,17 +57,18 @@
   Block sigValueBlock(sigValue, sizeof(sigValue));
 
   Signature sig(sigInfoBlock, sigValueBlock);
-  BOOST_REQUIRE_NO_THROW(SignatureSha256WithEcdsa(sig));
+  BOOST_CHECK_NO_THROW(SignatureSha256WithEcdsa(sig));
+  BOOST_CHECK_NO_THROW(sig.getKeyLocator());
 }
 
 BOOST_AUTO_TEST_CASE(Encoding)
 {
-  SignatureSha256WithEcdsa sig;
-
   Name name("/test/key/locator");
   KeyLocator keyLocator(name);
 
-  sig.setKeyLocator(keyLocator);
+  SignatureSha256WithEcdsa sig(keyLocator);
+
+  BOOST_CHECK_NO_THROW(sig.getKeyLocator());
 
   const Block& encodeSigInfoBlock = sig.getInfo();