bug 894462: servo build slaves need libpng. r=dustin

--HG--
rename : modules/packages/manifests/yasm-dmg.sh => modules/packages/manifests/libpng-dmg.sh
rename : modules/packages/manifests/yasm.pp => modules/packages/manifests/libpng.pp
bug 893867: transition to PuppetAgain based signing servers - enable signing4. r=rail
Bustage fix for new signing server: Add AWS master networks to allowed_ips. r=bustage
bug 893867: transition to PuppetAgain based signing servers. r=rail
Bug 892003: adds unixsock collectd plugin; r=dustin
bug 891870: use in-tree configs whenever possible for servo builds. r=bhearsum

--HG--
extra : rebase_source : e483c51bf84c13fbd9e25d62d07a921daf5c8572
Bustage fix for bug 891870: use custom renderer for mozharness_config interpolation, because we can't automatically render the value of a property.