Debian and its derivatives will autostart services when installing
packages. However, often we need to configure installed software
before it is ready to run, so it's preferable to use Salt states and
requisites to handle package installation or upgrades, configuration
changes and service [re]starting.

Adding a policy-rc.d file with exit code 101 sets a policy which denies
this default behavior. Making this change in the install script also
prevents new builders from attempting to contact the Salt master before
they have been configured properly.