Correct --unittest-only test behavior

run_tests.sh --unittest-only current issues a warning that
`muranoapi/tests/functional` can't be found, and tries to run the
tests in functionaltests, which require tempest and a running server.

Changes run_tests.sh to exclude `functionaltests` instead.

Change-Id: I86bca560ddf7906d43c0fcf80dd331163a22ea3e
Closes-Bug: 1303885
Merge "Correct --unittest-only test behavior"