Use a compute service type from tempest.conf

Auto negative test generator is using static service type 'compute',
but EndpointNotFound happens when customizing compute service type
on Keystone.
This patch fixes the problem by using a compute service type from
tempest.conf.

Change-Id: I5fcf53cc9a24bf4f2a00f11540bd87932212933f
Merge "Use a compute service type from tempest.conf"