flowprobe: fix sending L2 flows using L2_IP6 template

flowprobe: fix sending L2 flows using L2_IP6 template

Currently, L2 flows are exported using L2_IP6 template if L3 or L4
recording is enabled on L2 datapath. That occurs because during feature
enable, L2 template is added and its ID is not saved immediately. Then
L2_IP4 and L2_IP6 templates are added overwriting "template_id" each
time. And in the end, the current value of "template_id" is saved for L2
template. The problem is that "template_id" at that point contains the
ID of L2_IP6 template.

With this fix, save the template ID immediately after adding a template
for all variants (datapaths). Also, cover the case with a test.

Type: fix
Change-Id: Id27288043b3b8f0e89e77f45ae9a01fa7439e20e
Signed-off-by: Alexander Chernavin <achernavin@netgate.com>