systemd limits the number of process and threads you can create within the cgroup it creates for a given service. This is especially limiting if you have a high number of keys Nomad watches from Consul.
Fixes#4649
The default (control-group) kill mode in systemd will kill the associated executors, leading to a commonly seen behavior of nomad client restarts losing all current allocations.
setting the KillSignal to SIGINT seems to be a reasonable default, and allows things like leave_on_interrupt to function
The existing nomad.service is having minor configuration and this change
adds the following:
* Install section that allows nomad to be able enabled in systemd
* Default limit of open files is set to 65536
