Makefile: change confusing image and container names
Makefile: change confusing image and container names

The name 'apicast-xc' is problematic for 2 reasons:
  1) The targets in the Makefile are not really testing apicast, just the XC
     module.
  2) We have a Dockerfile to run Apicast with XC. In the README, we recommend
     to build it as 'apicast-xc'. I think that name makes sense in this case.
     As it collides with the ones we have in the Makefile, I think it is just
     better to change the ones in the Makefile.

3scale/apicast-xc