Make xenial-pike the default smoke test

Change-Id: I4b500716f08d2041044a204951750887e34fb87a

tox.ini

@@ -60,7 +60,7 @@ basepython = python2.7
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
-    bundletester -vl DEBUG -r json -o func-results.json gate-basic-xenial-mitaka --no-destroy
+    bundletester -vl DEBUG -r json -o func-results.json gate-basic-xenial-pike --no-destroy
 
 [testenv:func27-dfs]
 # Charm Functional Test