Upgrade to Tomcat 8.0.28 and test support for SSL config from classpath

Prior to 8.0.28 Tomcat required the key store and trust store (if any)
to be available directly on the filesystem, i.e. classpath: resources
would not work. Tomcat 8.0.28 removed this limitation.

This commit updates to Tomcat 8.0.28, updates the tests to verify
the new Tomcat capability and removes the obsolete documentation of
the restriction.

Closes gh-4048

spring-boot-samples/spring-boot-sample-tomcat-ssl/src/main/resources/application.properties

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