Fix coverage option for run_tests.sh

This commit fixes run_tests.sh so that when the -c flag is specified
coverage will actually be collected. Previously the --coverage option
was added to testrargs, however this isn't a testr flag. It is used by
pbr to wrap testr with coverage start and coverage report. This changes
run_tests() so that when coverage is specified it runs the unit tests
using setup.py so that the coverage flag can be used.

Change-Id: Iad0d508b6a4638a67e45646148144f627588fa48
Separate negative tests for test_multiple_create

Move negative tests from test_multiple_create.py to
test_multiple_create_negative.py.

Partially implements blueprint negative-test-files

Change-Id: I413d02356f2c3f578c01a6e2e7beb953576c4015
Merge "Fix coverage option for run_tests.sh"
Merge "Separate negative tests for test_multiple_create"