Restore ability to run functional tests with run_tests.sh

Change f20b1d42 which split the running of functional tests from
the running of unit tests caused running:
  ./run_tests.sh neutron.tests.functional
to fail as the OS_TEST_PATH variable will be defaulted to
neutron.tests.unit which means that the functional tests will not
be discovered in the above case.

This patch trys to detect an argument passed in the form of a
neutron.tests string and correctly sets the OS_TEST_PATH variable
based on it.

Change-Id: I8ccdc7f10d3c8478281aeaf8d02175d0eeb8d6e9
Updated from global requirements

Change-Id: I91778ac6d505f8808a359765555a1619c2f34d3c
Merge "Updated from global requirements"
Merge "Restore ability to run functional tests with run_tests.sh"