Inject build_flavor in track templates

[gbanasiak]

Here's an attempt at injecting build_flavor in track templates. It is part of Rally modifications for serverless.

The idea is to handle build_flavor as Jinja2 internal variable, similarly to now (doc).

Internal variables take precedence over user-supplied parameters and are excluded from a list of parameters exposed by a track (see here). As a result, today if a user provides now parameter via --track-params it is not recognised as a valid parameter even if it is present in a track definition. I found current error slightly misleading so this PR adds a dedicated error message when user-provided parameter collides with any of the variables defined internally.

Internal variables are not used today when rendering index settings, index templates, component templates and composable templates. This PR changes that adding build_flavor internal variable alone to these as well. That's necessary to support scenarios such as elastic/rally-tracks#406 where build_flavor could be used in component template definition.

With this approach build_flavor cannot be determined within custom runner or custom parameter source Python code. Their interface simply does not expose the necessary data structures. To workaround this we would need to inject build_flavor as an extra operation parameter which I did not find compelling. I don't think it's a blocker because build_flavor-driven customisation can still be achieved at the track definition level.

The build_flavor value is taken from Elasticsearch REST API call in an early stage of benchmark preparation flow, i.e. setup() in BenchmarkCoordinator (see #1748 diagram) and saved to configuration here. This call does not happen in case of source builds. If configuration entry is missing, default value of build_flavor is assumed.

This is a breaking change because build_flavor becomes reserved, so the plan is to release 2.8.1 and bump up version to 2.9.0 before merging it.

[dliappis]

The approach looks good to me and in tests I did it worked well. Left a very minor comment.

[ebadyano]

This might be a bit confusing, since the example only talks about now and days_ago. Not sure if we may want to add an example with bulld_flavour as well? and specify that the http_log example is for the other two?..

[gbanasiak]

Right, I haven't thought about this example below. I moved build_flavor variable description lower in 29351b4. I don't have an example that would make sense outside of serverless context, so I'm not adding it.

[ebadyano]

LGTM

[dliappis]

LGTM