Add Jetty SSL keystore in classpath test

Update JettyEmbeddedServletContainerFactoryTests to test loading an
SSL keystore from the classpath. Currently this is a Jetty only feature,
although if https://issues.apache.org/bugzilla/show_bug.cgi?id=56777 is
fixed we could do the same thing with Tomcat.

Closes gh-1943
Merge pull request #1943 from isopov/ssl-keystore-classpath-test

* ssl-keystore-classpath-test:
  Add Jetty SSL keystore in classpath test