default to our in-tree mock config when building on CentOS
default to our in-tree mock config when building on CentOS

scylla.git does a similar thing, albeit in a more complicated fashion,
testing for whether or not a rebuild is asked for and etc.

For us, the build process is a lot simpler, so it is better to just
point to the file when we detect that we are on CentOS and no explicit
target is given.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java,scylladb/scylla-tools-java