Fixes according to feedback in YARN-465 jira

1. Remove core-default.xml and provide necessary configuration in the test
itself
2. Improve logging messages
3. Add cookie checks
4. Remove non needed test method

Conflicts:
	hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-web-proxy/src/main/java/org/apache/hadoop/yarn/server/webproxy/WebAppProxyServer.java
	hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-web-proxy/src/test/java/org/apache/hadoop/yarn/server/webproxy/TestWebAppProxyServlet.java