This feature is already used on all the L3 devices, for consistency and leaner automation (currently behind flags) we should roll it to all the L2 devices as well once they're upgrades (only eqiad/codfw prod switches are left).
the fasw and asw1-eqsin switches didn't create the mgmt_junos routing instance as they should have.
https://gerrit.wikimedia.org/r/922161 works around the issue by adding a (hopefully) temporary variable to control the mgmt_junos routing instance.
Filtering on device type was not possible (as they're all asw) and filtering on device name/prefix was getting ugly as they're multiple devices impacted. This also allows to cleanly disable mgmt_junos on mr devices and on devices with no mgmt network (eg. knams routers)