Address review comments from allocations patch

This addresses some review comments from change
I699fd01d6dbd1c9c5186d1b86499ae2bae94286a:

1. Improve the help text wording for "allocation set" to
   let people know it's an overwrite/replacement of the
   existing allocations so they need to include old and new
   allocations if they don't want to lose the old ones when
   adding any new ones.

2. A couple of enhancements in the allocation unit tests.

Part of blueprint placement-osc-plugin

Change-Id: I4abebc709564d8afcbbe18e8d406f2c898d7ea8b

osc_placement/resources/allocation.py

@@ -44,7 +44,12 @@ def parse_allocations(allocation_strings):
 
 
 class SetAllocation(command.Lister):
-    """Set resource allocation(s) for a given consumer"""
+    """Replaces the set of resource allocation(s) for a given consumer
+
+    Note that this is a full replacement of the existing allocations. If you
+    want to retain the existing allocations and add a new resource class
+    allocation, you must specify all resource class allocations, old and new.
+    """
 
     def get_parser(self, prog_name):
         parser = super(SetAllocation, self).get_parser(prog_name)

osc_placement/tests/unit/test_allocation.py

@@ -10,14 +10,16 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
-import unittest
 import uuid
 
 from osc_lib import exceptions
+from oslotest import base
+import six
+
 from osc_placement.resources import allocation
 
 
-class TestAllocation(unittest.TestCase):
+class TestAllocation(base.BaseTestCase):
     def test_parse_allocations(self):
         rp1 = str(uuid.uuid4())
         rp2 = str(uuid.uuid4())
@@ -28,7 +30,8 @@ class TestAllocation(unittest.TestCase):
             rp1: {'VCPU': 4, 'MEMORY_MB': 16324},
             rp2: {'VCPU': 4, 'DISK_GB': 4096},
         }
-        self.assertEqual(expected, allocation.parse_allocations(allocations))
+        self.assertDictEqual(
+            expected, allocation.parse_allocations(allocations))
 
     def test_merge_allocations(self):
         rp1 = str(uuid.uuid4())
@@ -44,10 +47,13 @@ class TestAllocation(unittest.TestCase):
         allocations = [
             'rp={},VCPU=4,MEMORY_MB=16324'.format(rp1),
             'rp={},VCPU=8,DISK_GB=4096'.format(rp1)]
-        self.assertRaises(
+        ex = self.assertRaises(
             exceptions.CommandError, allocation.parse_allocations, allocations)
+        self.assertEqual(
+            'Conflict detected for resource provider %s resource class VCPU' %
+            rp1, six.text_type(ex))
 
-    def test_fail_if_incorect_format(self):
+    def test_fail_if_incorrect_format(self):
         allocations = ['incorrect_format']
         self.assertRaisesRegexp(
             ValueError,