Rename ubuntu-1604.json vars file

The packer-merge-ubuntu-16.04 job is expecting this format in order
to trigger the job automatically.

Change-Id: I9033c7b8863c1033f2deb1038d8506e009c7e3a4
Signed-off-by: Eric Ball <[email protected]>

diff --git a/packer/vars/ubuntu-1604.json b/packer/vars/ubuntu-16.04.json
similarity index 100%
rename from packer/vars/ubuntu-1604.json
rename to packer/vars/ubuntu-16.04.json