Allow dfs.data.transfer.protection default to hadoop.rpc.protection

Currently administrator needs to configure both dfs.data.transfer.protection and hadoop.rpc.protection to specify QOP for rpc and data transfer protocols. In some cases, the values for these two properties will be same. In those cases, it may be easier to allow dfs.data.transfer.protection default to hadoop.rpc.protection.
This also ensures that an admin will get QOP as Authentication if admin does not specify either of those values.

Separate jiras (HDFS-6858 and HDFS-6859) are created for dfs.data.transfer.saslproperties.resolver.class and dfs.data.transfer.protection respectively.