Allow pyvespa to set rank-features when defining rank-profile.

[thigm85]

Example:

rank-profile collect_rank_features inherits default {
	first-phase {
	    expression: random
	}
	ignore-default-rank-features
	rank-features {
	    bm25(title)
	    bm25(body)
	    nativeRank(title)
	    nativeRank(body)
	}
}

This is important when collecting training data for example.