511216: Support central USS OAuth login in MPC

Support USS 1.0.1 with regular login and USS 1.1 with OAuth

This reverts parts of the following commits, until all clients have
updated to USS 1.1:
- 1ca5fe5 Bug 499930 - MPC will need a client_id and client_secret for
Eclipse USS SDK
- 09b7551 499930: MPC will need a client_id and client_secret for
Eclipse USS SDK

Bug: 511216
Task-Url: https://bugs.eclipse.org/bugs/show_bug.cgi?id=511216
13 files changed