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-jetty-ssl/src/main/resources/application.properties

@@ -0,0 +1,4 @@
+server.port = 8443
+server.ssl.key-store = classpath:sample.jks
+server.ssl.key-store-password = secret
+server.ssl.key-password = password