The testing strategy doc was linking to the hacking repo docs
on creating unit tests, which are very specific to creating
unit tests for hacking rules.
This changes the link to the 'creating unit tests' section in
the HACKING.rst file, which has more information on testing
within nova.
Along with that change, the HACKING.rst testing section is
updated a bit to point out that we use stestr now instead of
testr and adds a proper link to the development environment
quickstart docs.
The nova/tests/unit/README.rst actually needs a lot of work,
but that's left for another day.
Change-Id: Ie5106d87d632286162b31ce132e947c306d21abd
Closes-Bug: #1732024
85235d229b