Ensure that headers are utf8, not unicode

Although the api.openstack.wsgi.ResponseObject converted the headers to
utf8 strings to comply with RFC for HTTP this was not enough, since some
responses that contained unicode strings were returned, causing an error
when running nova behind an http server like Apache. This change ensures
that headers are always an utf8 encoded string.

Closes-Bug: 1254017

Change-Id: I980326f1f15907bf57060acdd132011254289079
Merge "Ensure that headers are utf8, not unicode"