[SPARK-12056][CORE] Create a TaskAttemptContext only after calling setConf.

TaskAttemptContext's constructor will clone the configuration instead of referencing it. Calling setConf after creating TaskAttemptContext makes any changes to the configuration made inside setConf unperceived by RecordReader instances.

As an example, Titan's InputFormat will change conf when calling setConf. They wrap their InputFormat around Cassandra's ColumnFamilyInputFormat, and append Cassandra's configuration. This change fixes the following error when using Titan's CassandraInputFormat with Spark:

*java.lang.RuntimeException: org.apache.thrift.protocol.TProtocolException: Required field 'keyspace' was not present! Struct: set_key space_args(keyspace:null)*

There's a discussion of this error here: https://groups.google.com/forum/#!topic/aureliusgraphs/4zpwyrYbGAE

Author: Anderson de Andrade <adeandrade@verticalscope.com>

Closes #10046 from adeandrade/newhadooprdd-fix.