Include proper Content-Type in the HTTP Headers

Nova Metadata Service was not setting the Content-Type
in the HTTP Response headers. Because of this, clients
were receiving the default Content-Type which is "text/html".
The changes done in this patch will include proper Content-Type
in the HTTP response headers.

Closes-Bug: #1243878
Change-Id: Ia65cdf8099c85f63d7f44f93835c606d679a7ab3
Merge "Include proper Content-Type in the HTTP Headers"
Merge commit 'refs/changes/38/84138/2' of git://gold.eng.hq.xensource.com/git/github/openstack/nova into tmpbranch