Add gabbi tests for invalid date error

The create lease API returned a 500 response status code, but this is
fixed to return a 400 in https://review.openstack.org/583419/.

This patch adds a test for the fix.

Depends-On: https://review.openstack.org/583419/
Change-Id: Id289ae2259a03d49436294cdac1825acbd9005fb

blazar_tempest_plugin/tests/api/gabbits/leases.yaml

@@ -18,6 +18,16 @@ tests:
   response_strings:
     - End date must be later than start date.
 
+- name: post new lease with bogus start time
+  POST: reservation/v1/leases
+  data:
+    name: my_lease_wrong_time_order
+    start_date: "Bogus-date 13:13"
+    end_date: "2020-11-13 12:13"
+  status: 400
+  response_strings:
+    - "Bogus-date 13:13 is an invalid date. Required format: %Y-%m-%d %H:%M"
+
 - name: post new lease without end_date
   POST: reservation/v1/leases
   data: