ami: update packer to 1.0.0 so ENA gets enabled
ami: update packer to 1.0.0 so ENA gets enabled

Older versions of packer do not support ENA, and when faced
with the option

      "enhanced_networking": true,

will only actually enable it for the older 82599 VF instances.
Fortunately, packer 1.0 already supports it, and all we have to do is
update it.

While we are at it, let's check if the file is legit before using a
random file we have downloaded from the internet, to avoid breaching our
building process.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>
Message-Id: <20170406123952.14708-1-7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla