Add AWS SageMaker instructions

# Conflicts:
#	donkeycar/parts/datastore.py

Remove TensorFlow dependency

Small Documentation changes... 

Assume a compatible flavor of the tensorflow library is
pre-installed. This allows Donkey to be installed on systems
where tensorflow-gpu is installed w.o. having to fiddle with
dependencies.

Since all install instructions in Donkey docs pre-installs
tensorflow already, this is a minor change.

Add instructions to expand filesystem for the Pi

Update the disk image to use tensorflow 1.7 and support pi3b+

# Conflicts:
#	donkeycar/parts/datastore.py

Typo correction