Include proper Content-Type in the HTTP response headers

Include proper Content-Type in the HTTP response headers

Neutron namespace proxy handler and metadata agent were not setting the
Content-Type in its response. Both of them were returning only the response
data which is obtained from the nova-metadata-service. Since they were returning
only the response data, the Content-Type returned to the clients has the default
one which is - "text/html". Ideally this should be set to the data type which is
present in the HTTP Response. The fix now includes the Content-Type which is
returned by nova-metadata-service

Closes-Bug: #1243878
Change-Id: If68f0b508fbea4ecd1eb0e58d602b5ba6ccbe263