Adds mock ability to the ENI and Ifcfg providers

Specifying mock=True for 'apply' will return a string
representing the changes required, or commands to be used
in order to implement the requested/specified configuration.

This is also exposed to the cli, with -m (--mock), e.g.

os-net-config --mock -c ./etc/example_os_net_config_1.json -p eni