The default Linux Capabilities set enabled by the docker, exec, and
java task drivers includes CAP_NET_RAW (for making ping just work),
which has the side affect of opening an ARP DoS/MiTM attack between
tasks using bridge networking on the same host network.

https://docs.docker.com/engine/reference/run/#runtime-privilege-and-linux-capabilities

This PR disables CAP_NET_RAW for the docker, exec, and java task
drivers. The previous behavior can be restored for docker using the
allow_caps docker plugin configuration option.

A future version of nomad will enable similar configurability for the
exec and java task drivers.