CLI: hide deleted interfaces

Added a new interface flag - HIDDEN
Indicates that the interface does not appear in CLI/API.
Added three new interface functions:
vnet_sw_interface_is_api_visible - indicates if the sw_if_index should be displayed
vnet_swif_is_api_visible - variant for sw_interface
vnet_sw_interface_is_api_valid - tests if the given if_index exists and is visible
                                 for future use by api functions

Changed the unformat function to only accept visible interfaces
Changed vxlan to add the HIDDEN flag to deleted interfaces

This is the first part in a series to hide deleted interfaces from the API

Change-Id: Ib43cc5cf1c450856560faf4e84126eb3671038e2
Signed-off-by: Eyal Bari <ebari@cisco.com>