Removed the python module dependency checks and the selenium driver
package checks from configuration time tasks.

Updated all web tests to use new option to check dependencies at
runtime and raise a new exception in that case.

Updated the test of vtkListFiles protocol to only test for a couple
of small files which are now downloaded specifically in the
associated CMakeLists.txt for pvweb testing.  Also improved error
messages in the case of this test failing.

Change-Id: Ieeecfe33f07ad9c9782d3b9e1e5695dea58d9ac0