[Feature] Nomad constraint for consul maint mode

We would like there to be a scheduling constraint for when consul is in maintenance mode. A specific nomad client may get into a state where it can accept jobs but consul is in maintenance mode.

The affect then is if there are routing services like Fabio depending on the consul health checks the job is never routed to and considered down.

No files, tests, or entry points are named. Start by reviewing Nomad scheduling constraints and the Consul maintenance-mode integration, then consider how routing services such as Fabio use Consul health checks. Done means a client in Consul maintenance mode can be excluded from scheduling so dependent services do not route jobs to it.

Written by the indexing model from the issue text.