Update the link to firewall_block_image.img
Update the link to firewall_block_image.img

As the link http://205.177.226.237:9999/onosfw/firewall_block_image.img
has been deprecated, now the valid link is updated, aiming at running the
testcase onos_sfc.

JIRA: FUNCTEST-820

Change-Id: Id848e3ab92c3b0fb47e3c755d203844dd94d8408
Signed-off-by: Linda Wang <81613bebe84fa394bbc7c5cc1c21989c9bff2c52@huawei.com>

opnfv/functest,mywulin/functest,opnfv/functest,mywulin/functest