Pass configured/auth headers to a Datasource.

In some setups (ex openshift), the Datasource will require Grafana
to pass oauth token as header when sending queries.
Also, this PR allow to send any header which is something
Grafana currently does not support.

conf/provisioning/datasources/sample.yaml

@@ -40,11 +40,14 @@ apiVersion: 1
 #      graphiteVersion: "1.1"
 #      tlsAuth: true
 #      tlsAuthWithCACert: true
+#      httpHeaderName1: "Authorization"
 #   # <string> json object of data that will be encrypted.
 #   secureJsonData:
 #     tlsCACert: "..."
 #     tlsClientCert: "..."
 #     tlsClientKey: "..."
+#     # <openshift\kubernetes token example>
+#     httpHeaderValue1: "Bearer xf5yhfkpsnmgo"
 #   version: 1
 #   # <bool> allow users to edit datasources from the UI.
 #   editable: false