Copy network data only to hosts that need it

[bwachter]

For historical reasons currently every host has a network_data entry, which blows up the runtime data structures a lot. The majority of hosts don't need access to it at all, or only to a subset - with the inventory plugin this can be controlled now.

Add a host flag as well as a plugin default to import

• the full data structure
• a partial data structure containing of all hosts in the same groups - this is required for cluster setups to make sure a run happens on all cluster members
• a partial data structure containing only the host
• nothing