[202205] Clear /etc/resolv.conf after changing the mgmt interface configuration to static.

[oleksandrivantsiv]

Why I did it

fixes #12907

When the management interface IP address configuration changes from dynamic to static the DNS configuration (retrieved from the DHCP server) in /etc/resolv.conf remains uncleared. This leads to a DNS configuration pointing to the wrong nameserver.

How I did it

Use resolvconf package for managing DNS configuration. It is capable of tracking the source of DNS configuration and puts the configuration retrieved from the DHCP servers into a separate file. This allows implementation cleanup of DNS configuration retrieved from DHCP.

How to verify it

1. Ensure that the management interface has no static configuration.
2. Check that /etc/resolv.conf has DNS configuration.
3. Configure a static IP address on the management interface.
4. Verify that /etc/resolv.conf has no DNS configuration.
5. Remove the static IP address from the management interface.
6. Verify that /etc/resolv.conf has DNS configuration retrieved form DHCP server.

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012
• 202106
• 202111
• 202205
• 202211

Description for the changelog

Ensure to add label/tag for the feature raised. example - PR#2174 under sonic-utilities repo. where, Generic Config and Update feature has been labelled as GCU.

Link to config_db schema for YANG module changes

A picture of a cute animal (not mandatory but encouraged)