Do not display None in pretty tables for fields with no value
Do not display None in pretty tables for fields with no value

Fields that are present but unset in a backend (like the email field for
a user created with no email in the KVS backend with the keystone
command line) will appear as "None" when listed, instead of as an empty
field.

Change-Id: I86dc0a204847518e61ba9f6f46d4637d897cfac1

jamielennox/keystoneauth,citrix-openstack-build/keystoneauth,sileht/keystoneauth