Decrease number of submodules pulled into the user workspace

[taniazplv]

1. This instruction will result in pulling too many undesirable submodules into the user workspace:

2. cd DASH && git checkout dev-tests-extension
   git submodule update --init --recursive
   For example, it pulls in everything under dash-pipeline/SAI/SAI, e.g. SAI/bm which is old junk:

Cloning into '/home/chris/plvision-DASH/DASH/dash-pipeline/SAI/SAI/bm/behavioral-model'...

3. Notice the make target called sai-submodule. I use this to automatically pull in only the ptf submodule under SAI. You could use a similar approach to automatically and most important, selectively, pull in submodules you've added to the project. Please find some approach like this, thanks.
4. SAI/SAI: sai-submodule
   sai-submodule:
   @echo "Initializing SAI submodule..."
   git submodule update --init
   @echo "Initializing SAI submodule..."
   cd SAI/SAI/test && git submodule update --init ptf