Make Self-Host service run as Local System for nightly runs and PR's

[roncain]

Our nightly-build Self-Hosted services is being run as the local user, not Local System. Cross-machine testing has verified this causes problems with some authentication tests.

Proposed solution is:

1. Install PsExec.exe on the nightly build machine (and add to setup instructions for future reference)
2. Set the environment variable PSEXEC_PATH to the folder containing PsExec.exe

When this is done, the existing scripts to start the self-hosted service will run as Local System

[zhenlan]

For CI, because we run selfhosted service and tests on the same windows machine, this is not a problem.

For nightly lab run, we can enable this when we start the selfhosted service after we enable windows authentifcation on the server.