[SPARK-8851] [YARN] In Client mode, make sure the client logs in and updates tokens

In client side, the flow is SparkSubmit -> SparkContext -> yarn/Client. Since the yarn client only gets a cloned config and the staging dir is set here, it is not really possible to do re-logins in the SparkContext. So, do the initial logins in Spark Submit and do re-logins as we do now in the AM, but the Client behaves like an executor in this specific context and reads the credentials file to update the tokens. This way, even if the streaming context is started up from checkpoint - it is fine since we have logged in from SparkSubmit itself itself.

Author: Hari Shreedharan <hshreedharan@apache.org>

Closes #7394 from harishreedharan/yarn-client-login and squashes the following commits:

9a2166f [Hari Shreedharan] make it possible to use command line args and config parameters together.
de08f57 [Hari Shreedharan] Fix import order.
5c4fa63 [Hari Shreedharan] Add a comment explaining what is being done in YarnClientSchedulerBackend.
c872caa [Hari Shreedharan] Fix typo in log message.
2c80540 [Hari Shreedharan] Move token renewal to YarnClientSchedulerBackend.
0c48ac2 [Hari Shreedharan] Remove direct use of ExecutorDelegationTokenUpdater in Client.
26f8bfa [Hari Shreedharan] [SPARK-8851][YARN] In Client mode, make sure the client logs in and updates tokens.
58b1969 [Hari Shreedharan] Simple attempt 1.