Add variant override to updog arguments

[tjkirch]

If --variant is given, updog will use its argument to look for updates, rather
than the currently running variant. This allows for manually changing between
variants that you know are compatible.

Implementation note: running_version() was removed so that the current variant
isn't as easily used by accident when the user intended to override it.

Testing done:

Starting with a 1.15 AMI with this change:

[ec2-user@ip-192-168-11-222 ~]$ apiclient -u /
  "os": {
    "variant_id": "aws-k8s-1.15",
    ...

A 1.16 image in a test repo only shows up when --variant is specified:

bash-5.0# updog check-update -a --ignore-waves
bash-5.0# updog check-update -a --ignore-waves --variant aws-k8s-1.16
aws-k8s-1.16 0.3.2

Updates OK:

bash-5.0# updog update -i 0.3.2 --variant aws-k8s-1.16 -n -r
Starting update to 0.3.2
** Updating immediately **

...reboot, reconnect...

[ec2-user@ip-192-168-11-222 ~]$ apiclient  -u /
  "os": {
    "variant_id": "aws-k8s-1.16",
    ...

Instance is running, connects to my cluster, and runs a pod OK.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.