repoman: get default to update changelog from layout.conf

repoman: get default to update changelog from layout.conf

Updating the ChangeLog file may be desirable in more repos than just the
one named 'gentoo', like e.g. the Prefix one.  Hence, make this default
configurable though metadata/layout.conf.

This commit must go accompanied by a commit to
gentoo-x86/metadata/layout.conf that adds the following bit:

# Bug #337853 - gentoo's council says to enable
# --echangelog by default for the "gentoo" repo
update-changelog = true