Add mention of mounting cgroupfs properly to PACKAGERS.md

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Return error when existing bridge does not match ip
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)
Merge pull request #4613 from tianon/add-cgroupfs-mention-to-packagers

Add mention of mounting cgroupfs properly to PACKAGERS.md
Merge pull request #4871 from crosbymichael/bip-exists

Return error when existing bridge does not match ip
initial version of cli integration tests

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Small tweaks to the hack scripts to make them simpler

Please do with this as you please (including rebasing and/or squashing it), especially under clause (c) of the DCO.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)