BUG: Ensure extension Experimental targets are associated to Experimental track

Thanks for Ryan Eckbo developer of the "UKF extension" extension
for reporting and helping investigating the problem.

This regression is a side effect of commit r21682 [1] where parameters
are now written into files like:
  <extension-name>-test-command-args.cmake
  <extension-name>-upload-command-args.cmake
  <extension-name>-upload-only-command-args.cmake

And the file are then passed to the ctest using argument:

   -DSCRIPT_ARGS_FILE:FILEPATH=<path-to-args-file>

The written file are used to create custom targets:
  Experimenal .............. using EXTENSION_TEST_COMMAND
  ExperimentalUpload ....... using EXTENSION_UPLOAD_COMMAND
  ExperimentalUploadOnly ... using EXTENSION_UPLOAD_ONLY_COMMAND
  Nightly .................. using EXTENSION_TEST_COMMAND
  NightlyUpload ............ using EXTENSION_UPLOAD_COMMAND
  NightlyUploadOnly ........ using EXTENSION_UPLOAD_ONLY_COMMAND

Since the same files were used unconditionally for both Nightly and
Experimental targets, the latest value of CTEST_MODEL was used because
the latest set of files to be configured was done with CTEST_MODEL equal
to "Nightly".

This commit explicitly specify the value -DCTEST_MODEL:STRING=${CTEST_MODEL}
for each target command. Doing so ensure each command is associated with
the appropriate track.

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=21682

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21860 3bd1e089-480b-0410-8dfb-8563597acbee