Update RunPSScript to invoke scripts directly

PS scripts that are invoked via Invoke-Expression; will automatically
scope variable/function scopes to the script; so it's not necessary to
launch script within a new PowerShell process. This also has a benefit
of allowing debugger to break into setup scripts launched by LISA.
Update RunInitShutdownScript script invocation.

PS scripts that are invoked via Invoke-Expression; will automatically
scope variable/function scopes to the script; so it's not necessary to
launch script within a new PowerShell process. This also has a benefit
of allowing debugger to break into init/shutdown scripts launched by
LISA.
Merge remote-tracking branch 'upstream/master'
Merge pull request #89 from alexngmsft/master

Update script invocation to avoid creating a new PowerShell host and to ease debugging.