Adding support to use MPI based tests in Testing/External

There were a bunch of MPI tests failing when tests were built using
Testing/External. This was because MPI support was not brought in.

In VTK this support was brought in throught code in
Parallel/MPI/CMakeLists.txt. and since vtkMPIParallel was the root
modules for all MPI based modules it ensured that the environment was
brought in for every MPI based modules.

The MPI testing support logic was separated into another file named
vtkTestingMPISupport.cmake which was included at appropriate locations.

Change-Id: I2fa4cb023b77eb9aeb5c818e3625263b616ca42b
Merge topic 'include-vtkTestingExternal-MPISupport' into master

94d2cd0a Adding support to use MPI based tests in Testing/External
ENH: Use TCL_TCLSH versus VTK_TCL_EXE

For tcl tests that do no graphics there is no need to use
VTK_TCL_EXE. TCL_TCLSH works as long as the auto_path is set
corrrectly. This may help solve some timeouts on a few MAC builds.

Change-Id: I209e569af190dd90d78615fec92fb227149dbbc1
Merge topic 'TCLSHReplacesVTK' into master

9b78526a ENH: Use TCL_TCLSH versus VTK_TCL_EXE
COMP: Change to tclsh does not work on windows

The change to use tclsh rather than the vtk executable does not work
on windows.

This patch reverts that change.

Change-Id: I048e8c62a547a2a04a2923df084016b451c636d3
Merge topic 'BackoutTCLPatch' into master

13ab3b0e COMP: Change to tclsh does not work on windows