Remove compile time dependency on JDBC drivers [ENT-1363] (#387)

* Change JDBC drivers dependencies to integrationTestRuntimeOnly
* Added smoke test validating JDBC driver registration located in "./plugins" directory 
* Some docsite tweaks

client/rpc/src/smoke-test/java/net/corda/java/rpc/StandaloneCordaRPCJavaClientTest.java

@@ -49,7 +49,8 @@ public class StandaloneCordaRPCJavaClientTest {
             port.getAndIncrement(),
             true,
             Collections.singletonList(rpcUser),
-            true
+            true,
+            Collections.emptyList()
     );
 
     @Before