Add missing __init__.py file

Tempest API tests weren't discoverable.

Change-Id: Id21b480813d8afc4e6e9c11e4520b64e324fbfad
diff --git a/octavia_tempest_plugin/tests/v2/api/__init__.py b/octavia_tempest_plugin/tests/v2/api/__init__.py
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/octavia_tempest_plugin/tests/v2/api/__init__.py