Restore ability to run functional tests with run_tests.sh

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