Use response.text for returning unicode EC2 metadata

When serving meta-data that contains unicode, use the webob Response.text
instead of Response.body.  This also adds a test that triggers the bug in
addition to testing that all meta-data offered by the server can be served.

Closes-bug: #1380792

Change-Id: If2ddaa97bbc89cf574205c0327e3caa02dd503fc
(cherry picked from commit f0f07fdb6702f89f3800a5054e9365c3abc90f7a)
Merge "Use response.text for returning unicode EC2 metadata" into stable/juno