[entities] Move CWSource update_config method as a function in 3.15 migration script
This script is the only caller of this method. In the code of the function,
drop "skip_unknown" argument and continue accordingly upon OptionError.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile