Drastically simplify command to generate ninja files

Here is the command to generate an asan ninja file

python ./buildscripts/scons.py --variables-files=./etc/scons/mongodbtoolchain_stable_clang.vars --allocator=system --sanitize=address --link-model=dynamic --dbg --ninja generate-ninja ICECC=icecc CCACHE=ccache NINJA_PREFIX=asan VARIANT_DIR=asan --variables-files=etc/scons/developer_versions.vars 

Note that "--variables-files=etc/scons/developer_versions.vars" is on a trial run so we should not include it (for now)

We should make this command a one or two liner

Something like

./buildscripts/ninja-asan.py

An added benefit of this is that we get a layer of abstraction so we can change small things about the command but the result will be an ASAN build which is what a developer expects.