Add flavor extra spec validation tests

Tests cover the addition of flavor extra spec validation [1,2]. Test
validates the ability to create a flavor with extra specs using a custom
namespace. This change was introduced in 2.86 [3].

[1] https://blueprints.launchpad.net/nova/+spec/flavor-extra-spec-image-property-validation-extended
[2] https://review.opendev.org/c/openstack/nova-specs/+/682655/
[3] https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id79

Change-Id: I20267054af04da3753795f7008abcf1118a31bdd
2 files changed