Migrate tempest-account-generator to new cliff cli framework

Leave existing endpoint for now as to not disrupt existing
workflows and deprecate next cycle.

Add deprication warning for old usage.

Change-Id: I9db27a853184126a7f751800bc8cacd7dd4e40fd
Implements: blueprint tempest-cli-improvements
diff --git a/setup.cfg b/setup.cfg
index 183c1fb..cc3a365 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -33,6 +33,7 @@
     tempest-account-generator = tempest.cmd.account_generator:main
     tempest = tempest.cmd.main:main
 tempest.cm =
+    account-generator = tempest.cmd.account_generator:TempestAccountGenerator
     init = tempest.cmd.init:TempestInit
     cleanup = tempest.cmd.cleanup:TempestCleanup
     run-stress = tempest.cmd.run_stress:TempestRunStress