The default version of yarn is equal to the hadoop version

This is a part of [PR 590](https://github.com/apache/spark/pull/590)

Author: witgo <witgo@qq.com>

Closes #626 from witgo/yarn_version and squashes the following commits:

c390631 [witgo] restore  the yarn dependency declarations
f8a4ad8 [witgo] revert remove the dependency of avro in yarn-alpha
2df6cf5 [witgo] review commit
a1d876a [witgo] review commit
20e7e3e [witgo] review commit
c76763b [witgo] The default value of yarn.version is equal to hadoop.version
(cherry picked from commit fb0543224bcedb8ae3aab4a7ddcc6111a03378fe)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>