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
Remove unnecessary check for CONF.notify_on_state_change

Already checked CONF.notify_on_state_change at beginning, no
need to check it again

Change-Id: I757ff7b71bafdee9e2e6009e7489db520704c8cc
Merge "Include proper Content-Type in the HTTP Headers"
Merge "Remove unnecessary check for CONF.notify_on_state_change"