Add refresh_db flag to pkgrepo.managed

Though it's nice for the pkgrepo.managed to run apt-get update when
it adds a ppa, adding multiple ppa will cause it to be run multiple
times, which is slow. The refresh_db allows users to disable automatic
updates of the apt database, so that they can do so based on a wait
condition.
Make refresh_db conditional in the aptpkg module

aptpkg.mod_repo will call refresh_db when a repo is added, so it's
unnecessary for pkgrepo.manage to call refresh_db as well.
Add missing conditional for refresh_db in aptpkg.mod_repo
Merge pull request #13360 from lyft/refresh_db-flag-for-pkgrepo

Add refresh_db flag to pkgrepo.managed