Python/Pexpect: Fix winpexpect stub when not in default sys.path

WinPexpect uses a stub process to launch the child process with a clean
set of handles but needs to "import winpexpect" in the stub.  Teach the
stub to put the winpexpect location in sys.path before importing it in
case it is not there by default.

Change-Id: I6c770185f8bef9d3d4b695139ee710f6592c2f1c
Use Python/Pexpect from OSEHRAHelper and VistATestClient

Python/Pexpect now has both Pexpect and a WinPexpect sharing it.
Teach both OSEHRA modules that need Pexpect to use the modules
from this location if they are not installed on the system.
Remove Pexpect and WinPexpect from "Scripts/HowtoSetupEnv.rst"
since they are now provided automatically.

Change-Id: I19e989402811c9a6f46aba88c92baddcd5f5a859