Fix CMIS tests returning UpdateConflictException on checkout

ERROR: testSetContentStreamPWC (cmislibtest.DocumentTest)
Traceback (most recent call last):
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislibtest.py", line 1076, in testSetContentStreamPWC
    pwc = newDoc.checkout()
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/atompub/binding.py", line 2279, in checkout
    ATOM_XML_ENTRY_TYPE)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/atompub/binding.py", line 187, in post
    self._processCommonErrors(resp, url)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/cmis_services.py", line 62, in _processCommonErrors
    raise UpdateConflictException(error['status'], url)
UpdateConflictException: Error 409 at http://localhost:8080/nuxeo/atom/cmis/default/checkedout

ERROR: testCheckinAfterGetPWC (cmislibtest.DocumentTest)
Traceback (most recent call last):
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislibtest.py", line 911, in testCheckinAfterGetPWC
    testDoc.checkout()
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/atompub/binding.py", line 2279, in checkout
    ATOM_XML_ENTRY_TYPE)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/atompub/binding.py", line 187, in post
    self._processCommonErrors(resp, url)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/cmis_services.py", line 62, in _processCommonErrors
    raise UpdateConflictException(error['status'], url)
UpdateConflictException: Error 409 at http://localhost:8080/nuxeo/atom/cmis/default/checkedout

ERROR: testDeleteContentStreamPWC (cmislibtest.DocumentTest)
Traceback (most recent call last):
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislibtest.py", line 1198, in testDeleteContentStreamPWC
    pwc = newDoc.checkout()
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/atompub/binding.py", line 2279, in checkout
    ATOM_XML_ENTRY_TYPE)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/atompub/binding.py", line 187, in post
    self._processCommonErrors(resp, url)
  File "/opt/jenkins/workspace/master/FT-nuxeo-master-cmis-cap-tomcat/nuxeo-distribution/nuxeo-distribution-cap-cmis-tests/cmislib/cmis_services.py", line 62, in _processCommonErrors
    raise UpdateConflictException(error['status'], url)
UpdateConflictException: Error 409 at http://localhost:8080/nuxeo/atom/cmis/default/checkedout