Release 2019_009 (2019-04-04)

NixOS platform

• Provide redis4 role in addition to the redis role. The former installs version 4.x while the latter runs on the default version (currently 3.x) (#108957).

• Adapt Sensu server to accept check results from both 15.09 and 18.09 clients (#25409).

• Slow down release roll-out a bit to reduce infrastructure load during roll-out.

• Run check_disk plugin as root to ensure compatibility with docker mountpoints. (#107512)