OSS no longer needs the multi-file config Makefile content, so it was
removed. However, enterprise does benefit from having that around. We
can allow enterprise to expand on OSS' CircleCI Makefile by adding an
include directive in OSS. This will allow the Makefile to be the same on
both OSS and Enterprise, so merge conflicts cannot occur.

This adopts pattern used by Vault, where we split CircleCI yaml config
into multiple files that get packed and translated to 2.0.

This has two motivations: First, to ease translating config to CircleCI
2.0 so it can run on Enterprise private repository.  Second and most
importantly, it also adding Enterprise specific jobs in separate files
with reduced config file merging conflict resolution.