tests should use "clusterCompatibility" field from /pools/default to determine if cluster is 5.0

see test log here:
http://cv.jenkins.couchbase.com/job/couchdb-gerrit-views-pre-merge-master/label_exp=u16-docker-slave/579/consoleFull#-539669972712a389b-c7bc-46ec-bcae-02346ef3ce1b

Currently tests use "version" field to determine cluster compat mode, which is incorrect, since sometimes all nodes can be 5.0, but cluster still might be 4.6 (for example if the upgrade failed or during the upgrade)
http://src.couchbase.org/source/xref/trunk/testrunner/pytests/basetestcase.py#670

Also as you can see from the jenkins log above, our buildbot builds have version="0.0.0-0000-community" which breaks the test. (corresponding bug is MB-26894)

14:39:10 [2017-11-16 14:39:10,602] - [rest_client:1951] INFO - Node versions in cluster [u'0.0.0-0000-community']

14:39:10 [2017-11-16 14:39:10,602] - [basetestcase:2051] INFO - Atleast one of the nodes in the cluster is pre 5.0 version. Hence not creating rbac user for the cluster. RBAC is a 5.0 feature.

The correct thing to do is to use "clusterCompatibility" value. Which is an integer value (ex: 327681 for Vulcan). It is calculated as MajVer * 65536 + MinVer.

For Vulcan (5.1): 5 * 65536 + 1 = 327681