tempest is already defined in stackrc as default.
Without this definition in local.conf,
tempest is installed successfully.
If it still needs "enable_service tempest" definition
on local.conf,
devstack itself has some problem.
In my environment, tempest installation works
without this definition on local.conf.
Change-Id: I25cda0142538d21bb9656b471e65ca5b018e8378