cephadm: update code to use fetch_custom_config_files

cephadm: update code to use fetch_custom_config_files

Update the code to use fetch_custom_config_files rather than
get_custom_config_files.  Like other fetch_ functions recently added
to cephadm we don't LBYL for checking values on ctx, but rather
call the fetch function on ctx and then check if the result contains
actionable data.
get_custom_config_files gets removed as no remaining callers exist.

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