Fixing the delete vlan function

[tientienle]

Base on this link https://github.com/Azure/SONiC/wiki/Configuration, there are three tables involve when configuring vlan: VLAN, VLAN_MEMBER and VLAN_INTERFACE.
However, in the delete vlan function in https://github.com/Azure/sonic-utilities/blob/master/config/main.py, just two table are deleted, and the VLAN_INTERFACE has been left.

- What I did
Deleting VLAN_INTERFACE table as well when deleting a vlan.

- How I did it
Find the Vlan in VLAN_INTERFACE table and delete that table:
keys = [ (k, v) for k, v in db.get_table('VLAN_INTERFACE') if k == 'Vlan{}'.format(vid)]
for k in keys:
db.set_entry('VLAN_INTERFACE',k,None)

- How to verify it
Config a vlan with 3 tables: VLAN, VLAN_MEMBER and VLAN_INTERFACE as in https://github.com/Azure/SONiC/wiki/Configuration
Checking information in redis before and after delete vlan as below:

root@leaf2-10-58-0-22:/# redis-cli -n 4 keys *Vlan5*

1. "VLAN_INTERFACE|Vlan5|192.168.5.1/24"
2. "VLAN_MEMBER|Vlan5|Ethernet16"
3. "VLAN|Vlan5"

root@leaf2-10-58-0-22:/# config vlan del 5
root@leaf2-10-58-0-22:/# redis-cli -n 4 keys *Vlan5*
(empty list or set)
root@leaf2-10-58-0-22:/#

[jleveque]

Please make pull request title more clear and descriptive.

[tientienle]

Updated. Please let me know if anything needs to be more clarified.

[jleveque]

@tientienle: You did not make changes to the PR title. You updated the description. Please make the title more descriptive.

[stcheng]

@tientienle I believe right now the correct flow is to remove the VLAN interface IP first before removing the VLAN itself. What should be enhanced is to indicate the user that the current VLAN interface still has IP configured on it and shall not be removed directly. I don't think it is proper to directly remove the IPs associated with the VLAN in one shot.

[tientienle]

@stcheng Thanks for your comment. I've just checked the remove ip address in interface function. That makes sense now. So this is something that users need to be aware of.