RFE mock-core-configs have the same root for configurations that use the same core packages

[Sérgio Basto]

Hello,
As discussed on 
https://github.com/rpm-software-management/mock/pull/995#issuecomment-128...

So the request is have the same root for configurations that use the
same core packages and is more or less some subsets of packages. 

For example: centos-7-x86_64.cfg have have root centos-7-x86_64 (
config_opts['root'] ) and centos+epel-7-x86_64.cfg also will have the
same root i.e. just includes centos-7-x86_64.cfg and just added epel-7
repos .
Next, epel+rpmfusion_free-7-x86_64.cfg just includes centos+epel-7-
x86_64.cfg more rpmfusion_free repos and nonfree just includes free
more nonfree repos .

This also can be applied to epel-next i.e centos-stream+epel-next-9-
x86_64.cfg includes centos-stream+epel-9.cfg and centos-stream+epel-9-
x86_64.cfg includes centos-stream-9-x86_64.cfg

The bootstrap image will be the same and we will save a lot of disk
space not having different roots in /var/cache/mock/ and in
/var/lib/mock/ when we use mock for local builds with ccache enabled.

Best regards,
-- 
Sérgio M. B.