Fix yang model checking failed for MGMT_PORT

[kuanyu99]

What I did

Fix the configuration validation failure after I set the IP on the eth0 with the command "sudo config interface ip add eth0"

admin@sonic:~$ sudo config auto-techsupport global available-mem-threshold 20
libyang[0]: Leafref "/sonic-mgmt_port:sonic-mgmt_port/sonic-mgmt_port:MGMT_PORT/sonic-mgmt_port:MGMT_PORT_LIST/sonic-mgmt_port:name" of value "eth0" points to a non-existing leaf. (path: /sonic-mgmt_interface:sonic-mgmt_interface/MGMT_INTERFACE/MGMT_INTERFACE_LIST[name='eth0'][ip_prefix='192.168.3.30/16']/name)
libyang[0]: Leafref "/sonic-mgmt_port:sonic-mgmt_port/sonic-mgmt_port:MGMT_PORT/sonic-mgmt_port:MGMT_PORT_LIST/sonic-mgmt_port:name" of value "eth0" points to a non-existing leaf. (path: /sonic-mgmt_interface:sonic-mgmt_interface/MGMT_INTERFACE/MGMT_INTERFACE_LIST[name='eth0'][ip_prefix='192.168.3.30/16']/name)
sonic_yang(3):Data Loading Failed:Leafref "/sonic-mgmt_port:sonic-mgmt_port/sonic-mgmt_port:MGMT_PORT/sonic-mgmt_port:MGMT_PORT_LIST/sonic-mgmt_port:name" of value "eth0" points to a non-existing leaf.
Data Loading Failed
Leafref "/sonic-mgmt_port:sonic-mgmt_port/sonic-mgmt_port:MGMT_PORT/sonic-mgmt_port:MGMT_PORT_LIST/sonic-mgmt_port:name" of value "eth0" points to a non-existing leaf.
Error: Failed to validate configuration: ConfigMgmt Class creation failed
Aborted! 

How I did it

Add the MGMT_PORT config entry when I execute the CLI command to set the IP on the eth0 port.

How to verify it

1. Run a whole new sonic building procedure
2. Test the "config auto-techsupport" command after I setting the IP on the eth0
3. No configuration validation failure

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)