smoketest: interfaces: fix dhcpv6 pd testcase when using multiple interfaces
smoketest: interfaces: fix dhcpv6 pd testcase when using multiple interfaces

Commit a8e4317c ("smoketest: interfaces: test dhcpv6 pd sla-id auto increment")
added a new test, but when executed on multiple interfaces, e.g.:

TEST_ETH="eth1 eth2" /usr/libexec/vyos/tests/smoke/cli/test_interfaces_ethernet.py

A variable was not properly reset

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x