aws_instance: fix broken user_data()

[syuu1228]

user_data() is mistakenly unsupported IMDSv2, since it's not using __instance_metadata().

Fixed the code to use __instance_metadata().

Fixes #498

[syuu1228]

Tested at https://jenkins.scylladb.com/view/releng-testing/job/scylla-master/job/releng-testing/job/next-machine-image/280/

[yaronkaikov]

LGTM

[syuu1228]

@yaronkaikov The patch is already merged though, I have one thing don't understand.

When I testing this patch with IMDSv2 mode userdata, I used example code of the userdata from our README.md.
Witch is something like this:

scylla_yaml:
  cluster_name: test-cluster
  experimental: true
  seed_provider:
    - class_name: org.apache.cassandra.locator.SimpleSeedProvider
      parameters:
        - seeds: 10.0.219.209
post_configuration_script: "#! /bin/bash\nyum install cloud-init-cfn"
start_scylla_on_first_boot: true

(The script was for CentOS7 so I modified the command to apt-get, but anyway)

But I realized that it causes "binascii.Error: Incorrect padding" exception while loading post_configuration_script.
Because our script expected post_configuration_script is base64 encoded, but the example YAML code doesn't.

So it only accepts something like this:

scylla_yaml:
  cluster_name: test-cluster
  experimental: true
  seed_provider:
    - class_name: org.apache.cassandra.locator.SimpleSeedProvider
      parameters:
        - seeds: 10.0.219.209
post_configuration_script: IyEgL2Jpbi9iYXNoCnl1bSBpbnN0YWxsIGNsb3VkLWluaXQtY2Zu # base64 encoded
start_scylla_on_first_boot: true

Entire userdata will encode in base64 anyway (at least on EC2 I think), so post_configuration_script will encoded twice.
I'm not sure this is correct behavior since our example code does not have such thing, so I added try-except condition on my patch to both:
https://github.com/scylladb/scylla-machine-image/pull/507/files#diff-71a59e03175d0b8ae7712c9a6ddc7bc6daeb902291a81b2f40ef59dda4d344f9R118

But I actually found both SCT and testcase in scylla-machine-image expected post_configuration_script is encoded in base64.

Do you know which way is correct?
If base64 encoded is the correct way, I will send new patch to drop my changes.

[fruch]

post_configuration_script should be base64 encoded
is always was since the days it was a command line argument to cassandra scripts

it might be a nice extension feature to support also plain text, since now we are using yaml/json, but the base64 should still be working.