bug fix #257: Specify the mime type for debugreports static dir

When no mime type is configured for cherrypy static dirs, cherrypy uses python
mimetypes module to guess the corret mime type based on file extension.
Then mimetypes modules checks some configuration files such as /etc/mime.types
to get the corret mime type to a given file extension.
But on Ubuntu there is no entry to .xz extension so cherrypy assumes text/html
Because that when the user try to download a debugreport from kimchi it try to
open it in another browser tab.
So add the correct mime type for .xz extension for debugreports static dir in
cherrypy configuration.

Reference: http://docs.cherrypy.org/stable/progguide/files/static.html#specify-\
           the-content-type-of-static-resource

Signed-off-by: Aline Manera <alinefm@br.ibm.com>
Signed-off-by: Adam King <rak@linux.vnet.ibm.com>