[SPARK-20079][YARN] Fix client AM not allocating executors after restart.

The main goal of this change is to avoid the situation described
in the bug, where an AM restart in the middle of a job may cause
no new executors to be allocated because of faulty logic in the
reset path.

The change does two things:

- fixes the executor alloc manager's reset() so that it does not
  stop allocation after a reset() in the middle of a job
- re-orders the initialization of the YarnAllocator class so that
  it fetches the current executor ID before triggering the reset()
  above.

This ensures both that the new allocator gets new requests for executors,
and that it starts from the correct executor id.

Tested with unit tests and by manually causing AM restarts while
running jobs using spark-shell in YARN mode.

Closes #17882

Author: Marcelo Vanzin <vanzin@cloudera.com>
Author: Guoqiang Li <witgo@qq.com>

Closes #18663 from vanzin/SPARK-20079.