Systemd limits issue

[eliranbz]

Under systemd, the nofile and nproc should be set within the consul.service in order for them to take effect.
Under heavy installations, there are multiple errors such as:

[ERR] memberlist: Error accepting TCP connection: accept tcp [::]:8301: accept4: too many open files
[ERR] consul.rpc: failed to accept RPC conn: accept tcp [::]:8300: accept4: too many open files
[ERR] memberlist: Error accepting TCP connection: accept tcp [::]:8301: accept4: too many open files
[ERR] consul.rpc: failed to accept RPC conn: accept tcp [::]:8300: accept4: too many open files
[ERR] memberlist: Error accepting TCP connection: accept tcp [::]:8302: accept4: too many open files
[ERR] memberlist: Error accepting TCP connection: accept tcp [::]:8302: accept4: too many open files [ERR] memberlist: Error accepting TCP connection: accept tcp [::]:8301: accept4: too many open files
[ERR] memberlist: Error accepting TCP connection: accept tcp [::]:8301: accept4: too many open files
[ERR] consul.rpc: failed to accept RPC conn: accept tcp [::]:8300: accept4: too many open files

It's worth mentioning that systemd ignores the global/user limits configuration such as sysctl.conf and limits.conf
I think we should add this to the service, will open a pull request