Setting default valgrind correctly, allowing valgrind options override (Trilinos #6252)

Now, CTEST_MEMORYCHECK_COMMAND will automatically get set to the 'valgrind' in
the default env using FIND_PROGRAM().

You can also now override CTEST_MEMORYCHECK_COMMAND_OPTIONS in the env and
play with different options when using:

  $ env CTEST_MEMORYCHECK_COMMAND_OPTIONS="???" make dashboard

This is helping to look at the memory leak with AztecOO (see Trilinos #6252).