cephadm: add fetch_custom_config_files function

cephadm: add fetch_custom_config_files function

This function works similarly to get_custom_config_files but doesn't
require the use of "ctx.config_json" and will prefer sourcing the
data from `config_blobs`.
This function also returns the list of dicts rather than return a dict
with exactly one key which maps to said list of dicts. There appeared
to be no benefit to that when the call sites of get_custom_config_files
was examined.

This function is `fetch_configs` evil twin.

Signed-off-by: John Mulligan <jmulligan@redhat.com>