SPARK-1121: Include avro for yarn-alpha builds

This lets us explicitly include Avro based on a profile for 0.23.X
builds. It makes me sad how convoluted it is to express this logic
in Maven. @tgraves and @sryza curious if this works for you.

I'm also considering just reverting to how it was before. The only
real problem was that Spark advertised a dependency on Avro
even though it only really depends transitively on Avro through
other deps.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #49 from pwendell/avro-build-fix and squashes the following commits:

8d6ee92 [Patrick Wendell] SPARK-1121: Add avro to yarn-alpha profile
Add Jekyll tag to isolate "production-only" doc components.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #56 from pwendell/jekyll-prod and squashes the following commits:

1bdc3a8 [Patrick Wendell] Add Jekyll tag to isolate "production-only" doc components.
Merge branch 'master' of https://github.com/apache/spark