Allow dfs.data.transfer.saslproperties.resolver.class default to hadoop.security.saslproperties.resolver.class

Currently administrator needs to configure both

dfs.data.transfer.saslproperties.resolver.class

and

hadoop.security.saslproperties.resolver.class

to override saslproperties for rpc and data transfer properties. In some cases, the values for these two properties will be same. In those cases, it may be easier if

dfs.data.transfer.saslproperties.resolver.class

defaults to

hadoop.security.saslproperties.resolver.class

.