rgw: Fix the run-s3tests.sh run in build dir

[inevity]

Fixes http://tracker.ceph.com/issues/22141
When run ../qa/workunits/rgw/run-s3tests.sh in ceph build dir,
s3tests.fuzz.test.test_fuzzer.test_load_grap need open
file request_decision_graph.yml,but cannot found as
"1623 graph_file = open('request_decision_graph.yml', 'r')
1624 IOError: [Errno 2] No such file or directory:
'request_decision_graph.yml'"

When run ../qa/workunits/rgw/run-s3tests.sh in ceph build dir, if
terminate the running script,leave the tmp s3-test dir exist.
Now if we again run this script,nosetests will find too many
testcase in previous tmp s3-test dir, and will have relative
import module error such as 'Failure: ImportError
(cannot import name realistic) ... ERROR'

Signed-off-by: baul roidinev@gmail.com

[inevity]

@cbodley please help review!

[theanalyst]

jenkins test this please

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