by moving ConfigValues out of md_config_t, we are able to point
md_config_t to a new ConfigValues instance at runtime, without
destroying the old one.
also remove ConfigReader, because all md_config_obs_t will be updated
with ConfigProxy now. as md_config_t is not able to offer the *data*
anymore.
Signed-off-by: Kefu Chai <kchai@redhat.com>