[placement] Update allocation-candidates.yaml for gabbi 1.35

Gabbi 1.35 provides template variable susbtitution on the left hand side
of a JSONPath expression. That's used here to remove the need for static
fake uuids in the test data for allocation-candidates and the
SharedStorageFixture.

A couple of other clean ups to the use of gabbi, unrelated to the 1.35
update:

* It's possible to test the entire value in one response:

    resources:
         VCPU: 4
         DISK_GB: 5

  instead of:

    resources.VCPU: 4
    resources.DISK_GB: 5

  This is handy when the root of the expression is calculated and
  repeating that calculation is noisy.

* [?foo.bar=baz] is the more canonical form of [?(@.foo.bar=baz)]. The
  former is built into jsonpath, the latter dips into the underlying
  implementation.

Change-Id: Ie6b5e0186533581815d04411ed7b321948464bb6

test-requirements.txt

@@ -26,7 +26,7 @@ testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=1.4.0 # MIT
 bandit>=1.1.0 # Apache-2.0
 openstackdocstheme>=1.11.0 # Apache-2.0
-gabbi>=1.30.0 # Apache-2.0
+gabbi>=1.35.0 # Apache-2.0
 
 # vmwareapi driver specific dependencies
 oslo.vmware>=2.17.0 # Apache-2.0