Updates the documentation for nova unit tests

unit_tests.rst is outdated and does not reflect the recent changes
in the run_tests.sh script. Fix required an update of the usage
output of run_tests.sh as well as changes to some references.
Nova now uses testrepository and flake8.

There were also changes to the examples shown in the section
"Running a subset of tests". All the updated examples were tested.

Removes white spaces at the of end 3 lines.

Change-Id: I3fa040d5682ad94d861cef2a64b19b4da947318b
Closes-Bug: #1219040