suite/run.py: refactor to reuse config substitution code
suite/run.py: refactor to reuse config substitution code

In --newest mode we must recreate the base config, because
the ceph sha1 has been substituted several times already.
Factor the substitution code out of create_initial_config so
that it can be called from the backtracking code (which requires
storing kernel_dict and config_input on the instance).

Signed-off-by: Dan Mick <b07550071eaa6a9296289c43bbd6c90559196431@redhat.com>

SUSE/teuthology,ktdreyer/teuthology,ceph/teuthology,ceph/teuthology,SUSE/teuthology,ktdreyer/teuthology,SUSE/teuthology,dmick/teuthology,dmick/teuthology,dmick/teuthology