ENH: Update dashboard driver to support customization using env variable

After a nightly or continuous dashboard starts, it not uncommon to
(1) first stop the script and then (2) manually edit the script and driver
script to skip the directory cleaning part and (3) finally, after
fixing something, relaunching the script.

This commit introduces the variable "run_ctest_with_disable_clean"
set to FALSE by default. It basically allows to explicitly disable the
cleaning step.

Finally, the macro "setOnlyIfNotDefined" has been updated to also
check if the variable name is set in the environment. For more details,
refer to the inlined comments. This will basically allow to restart a
nightly build without (1) cleaning the build directory
and (2) without having to edit the dashboard files.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23417 3bd1e089-480b-0410-8dfb-8563597acbee