Before this change if an extra config file had new keys they would get
merged to the main config but if it contained an existing key, like
'providers' it would overwrite the original config.
Adding a library 'deep_merge' to do a more natural merge, where existing keys
get sub-elements added together and arrays are combined like for the
pool configuration.
Adding spec tests around EXTRA_CONFIG as they were missing, by adding
and testing two new extra_config.yaml fixture files
