Use variables to configure node index parameter in aliasByNode

[gajus]

At the moment, all the metric queries use hardcoded node index number to invokealiasByNode function, e.g.

aliasByNode(sitespeed_io.$path.summary.$group.$browser.$connectivity.browsertime.pageTimings.*.median, 8);

Here is an example in the code base,

grafana-bootstrap-docker/dashboards/pageSummary.json

Line 627 in b04d279

"target": "aliasByNode(sitespeed_io.$path.pageSummary.$domain.$page.$browser.$connectivity.browsertime.statistics.timings.pageTimings.*.median, 11)",

This makes it very tedious to add new dimensions (e.g. such as this requirement sitespeedio/sitespeed.io#1327).

Consider using variables, e.g.

aliasByNode(sitespeed_io.$path.summary.$group.$browser.$connectivity.browsertime.pageTimings.*.median, $aliasNodeDepth);

[gajus]

Or even smarter approach would be to use a constant that defines a number of new dimensions after the default namespace, default being 1 and add this constant to whatever the metric depth is. Then if user adds a new depth to the data, user would only need to increment the former variable.