[QENG-4075] Allow specifying 'graphs:' for dashboard

Prior to this the dashboard front-end would use the configuration settings
for `graphite[:server]`/`graphite[:prefix]` to locate a graphite server
to use for rendering graphs.

Now that we have multiple possible metrics backends, the front-end graph
host for the dashboard could be entirely different from the back-end metrics
server that we publish to (if any).

This decouples those settings:

 - use `graphs[:server]` / `graphs[:prefix]` for the graphite-compatible web front-end to use for dashboard display graphs
 - fall back to `graphite[:server]`/`graphite[:prefix]` if `graphs` is not specified, in order to support legacy `vmpooler.yaml` configurations.

Note that since `statsd` takes precedence over `graphite`, it's possible to specify both `statsd` (for publishing) and `graphite` (for reading). We still prefer `graphs` over `graphite`.

Updated the example `vmpooler.yaml` config file.

vmpooler.yaml.example

@@ -54,6 +54,32 @@
   server: 'redis.company.com'
 
 
+  # :graphs:
+  #
+  # This section contains the server and prefix information for a graphite-
+  # compatible web front-end where graphs may be viewed. This is used by the
+  # vmpooler dashboard to retrieve statistics and graphs for a given instance.
+  #
+  # NOTE: This is not the endpoint for publishing metrics data. See `graphite:`
+  #       and `statsd:` below.
+  #
+  # NOTE: If `graphs:` is not set, for legacy compatibility, `graphite:` will be
+  #       consulted for `server`/`prefix` information to use in locating a
+  #       graph server for our dashboard.  `graphs:` is recommended over
+  #       `graphite:`
+  #
+  #
+  # Available configuration parameters:
+  #
+  #
+  #   - server
+  #     The FQDN hostname of the statsd daemon.
+  #     (required)
+  #
+  #   - prefix
+  #     The prefix to use while storing statsd data.
+  #     (optional; default: 'vmpooler')
+
   # :statsd:
   #
   # This section contains the connection information required to store