Currently, all unknown configuration options are passed to Netmiko.  This
allows to pass Netmiko-specific options easily from the NGS configuration.
However, if Netmiko does not recognize an option, it crashes with an error
such as: `__init__() got an unexpected keyword argument 'ngs_foo'`

We now ignore any option that starts with `ngs_` to help with
compatibility (old NGS with new config) and we print a warning instead of
crashing.

To give more context in this warning, we now pass the device name (as
specified in the configuration) to the device class itself.

Change-Id: Ife4e15d2c0462337b0a88a83a3ea0aef6fb78a41