[bug 1112084] create requirements/dev.txt

Create a requirements/dev.txt and specify the dev requirements in it
[bug 1112084] Update provisioning script to install dev requirements

Since we now have a requirements/dev.txt listing all the dev
requirements, we can install those requirements while provisioning
the VM. This is a better way to fix bug 1075563.
Remove instructions for manual installation of flake8

flake8 is automatically installed while the VM is provisioned