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
Use --noop instead of --mock.

Updates the apply() function for each NetConfig object
so that it now accepts noop instead. Also, the updated
files are not returned (always instead of conditionally
if mock was used).
Add configuration samples.