Issue in PortChannel - Removing given vlan members from port channel #76

[rahul-1475]

Fixes #76

Issues :

Adding addtional functionaltity to removing given vlan members from port channel.

• code to deleting given access / trunk vlans.
• implementing in test cases to check it.

Fixes :

• modifying code to delete port channel vlan members.
• added function to delete mutiple delete requests - get_gnmi_del_reqs
• added test case to delete access valn or trunk vlan and both.

[rahul-1475]

orca_backend pr: https://github.com/STORDIS/orca_backend/pull/48