Multi asic platform changes for interface, portchannel commands (sonic-net#878)

* Initial changes for interface, portchannel commands to support multi-asic

* Updates based on the move of common API's to sonic_device_util.py

* Using the namespace as string instead of unicode. Additionaly changes to update the
namespace keyword to ctx.obj['namespace'] in portconfig scripts invocation.

* Fix in sfputil scripts for config interface transceiver commands

* Interface range not supported in multi-asic platforms - check added for now.
   Will add this support in future.

* Updates for sonic-py-common TODO's

* Updates to take care of following
   1. Make namespace option "required" for interface commands also, in addition to portchannel, vlan commands
   2. The namespaces choice is given to user to select the namespace option.

* Updates includes the following
  1. The API's interface_alias_to_name(), interface_name_is_valid(), interface_name_to_alias(),
     to handle the case when the input config_db is passed as "None"
  2. The API get_port_namespace() API to derive the namespace if the input interface name is in alias mode.

* Vlan command moved to a different file vlan.py, hence removed from this change set.

* Fixes needed to get in sync with the common library changes.

Author: judyjoseph