Add quotes around keys in exporter config template

These quotes are added to fix the pydantic error caused while reading
the yaml file. If the quotes are absent for the redfish keys, the
default value of empty string is registered as a None type while using
safe_load and reading the yaml file in prometheus-hardware-exporter.

Fixes canonical#43

templates/hardware-exporter-config.yaml.j2

@@ -8,7 +8,7 @@ enable_collectors:
 {% endif %}
 
 {% if REDFISH_ENABLE  %}
-redfish_host: {{ REDFISH_HOST }}
-redfish_username: {{ REDFISH_USERNAME }}
-redfish_password: {{ REDFISH_PASSWORD }}
+redfish_host: "{{ REDFISH_HOST }}"
+redfish_username: "{{ REDFISH_USERNAME }}"
+redfish_password: "{{ REDFISH_PASSWORD }}"
 {% endif %}