ifconfig: T6972: smoketests fail as IP address is not removed in time

From time to time integration tests fail as the DHCP assigned IP address is not
removed in time then dhclient stops. Add an explicit code path cleaning dynamic
assigned addresses from interface when disabling DHCP - if such a dynamic
address is remaining.

FAIL: test_dhcp_vrf (__main__.EthernetInterfaceTest.test_dhcp_vrf)

Traceback (most recent call last):

File "/usr/libexec/vyos/tests/smoke/cli/test_interfaces_ethernet.py", line 72, in tearDown
  self.assertNotIn(AF_INET, ifaddresses(interface))

AssertionError: 2 unexpectedly found in {17: [{'addr': '52:54:00:00:00:00',

'broadcast': 'ff:ff:ff:ff:ff:ff'}], 2: [{'addr': '192.0.2.103', 'netmask': '255.255.255.0',
'broadcast': '192.0.2.255'}], 10: [{'addr': 'fe80::5054:ff:fe00:0%eth0',
'netmask': 'ffff:ffff:ffff:ffff::/64'}]}