Fix flavor_client create_flavor interface

In flavor_clients compute service client, create_flavor
method does not allow to pass 0/none/false etc for
'OS-FLV-EXT-DATA:ephemeral' and 'os-flavor-access:is_public'.

https://github.com/openstack/tempest/blob/d12c233343aa490a23b9ffd3a58c5df6ff2bf9da/tempest/lib/services/compute/flavors_client.py#L70-L72

Code should check existence if those in kwargs instead of
their value and then populate the request body arg.

Change-Id: I0fda1aa2d126e10c986bfe445637d61e5b60de2d
Closes-Bug: #1650418
diff --git a/tempest/lib/services/compute/flavors_client.py b/tempest/lib/services/compute/flavors_client.py
index 4d1044b..a83c68b 100644
--- a/tempest/lib/services/compute/flavors_client.py
+++ b/tempest/lib/services/compute/flavors_client.py
@@ -67,9 +67,9 @@
         API reference:
         http://developer.openstack.org/api-ref-compute-v2.1.html#createFlavor
         """
-        if kwargs.get('ephemeral'):
+        if 'ephemeral' in kwargs:
             kwargs['OS-FLV-EXT-DATA:ephemeral'] = kwargs.pop('ephemeral')
-        if kwargs.get('is_public'):
+        if 'is_public' in kwargs:
             kwargs['os-flavor-access:is_public'] = kwargs.pop('is_public')
 
         post_body = json.dumps({'flavor': kwargs})