client: bring the defaultNetworkManager functionality inline with Docker driver network implementation

[jrasell]

PR #11173 enhanced the job specification to allow setting the hostname of a network namespace. This hostname can currently only be used when using the Docker driver. Nomad should be enhanced so that this parameter can be utilised on any driver which utilises the defaultNetworkManager such as exec and Java. External plugins that must initiate the network will need to implement this change also; if you have any questions please feel free to reach out.

The defaultNetworkManager implementation requires additional work than was needed of the Docker driver in order to make this functionality viable, and is the reason the work has been targeted as a followup item.

• allow setting the hostname of the network namespace, accordingly to the user supplied parameter or lack of
• populate the /etc/hostname file of the network namespace to match that of the hostname set in the previous action
• populated the /etc/hosts file of the network namespace to include the hostname set in action 1