PTFM-12436 Update code that previously used auth.user_info.

Summary:
Some dx commands and tests no longer need authserver for getting user_info.

* dx whoami does not query authserver.
* Added new tests for "dx whoami".
* ssh_config and verify_ssh_config do not query authserver.
* test_dx_ssh_config and test_dx_ssh no longer need user_info to get the
  current user's pubkey.
* test_dx_upload_with_upload_perm no longer needs user_info to decrease
  perms for the current user.
* Various app tests no longer need user_info to set the billTo on apps.

Test Plan:
added new tests for "dx whoami". Other affected subcommands have
existing tests.

Reviewers: klai, orodeh

Differential Revision: https://phabricator.dnanexus.com/D34