Restore compatibility with Jetty 8 when SSL is being used

ServerConnector is in different packages in Jetty 8 and Jetty 9 which
was leading to a NoClassDefFounderError when SSL was used with
Jetty 8.

This commit updates SslServerConnectorFactory to return an
AbstractConnector, a super class of ServerConnector, that is in the
same package in both Jetty 8 and Jetty 9. This class does not provide
a setPort method so the setting of the port has been pushed down into
the SslServerConnectorFactory implementation.

SSL samples for both Jetty 8 and Jetty 9 have been added to verify
SSL with both supported versions of Jetty.

Closes gh-1925

spring-boot-samples/spring-boot-sample-jetty8/src/test/java/sample/jetty/SampleJetty8ApplicationTests.java

@@ -40,7 +40,7 @@ import static org.junit.Assert.assertEquals;
 @WebAppConfiguration
 @IntegrationTest("server.port:0")
 @DirtiesContext
-public class SampleJettyApplicationTests {
+public class SampleJetty8ApplicationTests {
 
 	@Value("${local.server.port}")
 	private int port;