adding support for basic authentication without roles

This is initial patch for adding basic authentication support to plugin. I have tested it in my setup. It has added to config parameters as

security.basic.mode: true
security.basic.users.file: "/etc/elasticsearch/users.xml"

and users.xml should look like

<tomcat-users>
  <user name="test" password="tomcat" roles="tomcat" />
</tomcat-users>

Currently it does not test roles as it is set to authOnly.