Use correct CONFIG variable in LFC test.

Use correct CONFIG variable in LFC test.

Before change (IPv4):
===
INFO/test_lib: wait_for_message DHCPSRV_MEMFILE_LFC_EXECUTE: ...........
ERROR: Server did not execute LFC.
[...]
2016-09-25 14:02:28.988 INFO  [kea-dhcp4.dhcpsrv/53382] DHCPSRV_MEMFILE_DB opening memory file lease database: lfc-interval=1 name=/home/ports/pobj/kea-1.1.0-beta/kea-1.1.0-beta/src/bin/dhcp4/tests/test_leases.csv persist=true type=memfile universe=4
[...]
2016-09-25 14:02:31.092 INFO  [kea-dhcp4.dhcpsrv/53382] DHCPSRV_MEMFILE_DB opening memory file lease database: lfc-interval=0 name=/home/ports/pobj/kea-1.1.0-beta/kea-1.1.0-beta/src/bin/dhcp4/tests/test_leases.csv persist=false type=memfile universe=4
[...]
===

Before change (IPv6):
===
INFO/test_lib: wait_for_message DHCPSRV_MEMFILE_LFC_EXECUTE: ...........
ERROR: Server did not execute LFC.
[...]
2016-09-25 14:03:46.249 INFO  [kea-dhcp6.dhcpsrv/44306] DHCPSRV_MEMFILE_DB opening memory file lease database: lfc-interval=1 name=/home/ports/pobj/kea-1.1.0-beta/kea-1.1.0-beta/src/bin/dhcp6/tests/test_leases.csv persist=true type=memfile universe=6
[...]
2016-09-25 14:03:48.333 INFO  [kea-dhcp6.dhcpsrv/44306] DHCPSRV_MEMFILE_DB opening memory file lease database: lfc-interval=0 name=/home/ports/pobj/kea-1.1.0-beta/kea-1.1.0-beta/src/bin/dhcp6/tests/test_leases.csv persist=false type=memfile universe=6
[...]
===