capi: support Flatcar for AWS AMI

[dongsupark]

Support AWS AMI as well for Flatcar Container Linux.
At the moment it only supports Flatcar stable.

There are a couple of on-going issues.

• pip installation was disabled, because of unknown errors when running pip install for cloudformation and awscli.
• goss checks were also disabled for Flatcar, as cloud-init field is not detected at all.

In addition, a new env variable was added CAPI_PROVIDER for hack/image-build-flatcar.sh.

To test with AWS, first, set up your AWS account described here.

Then run like that:

cd images/capi
CAPI_PROVIDER="ami" ./hack/image-build-flatcar.sh

[t-lo]

Did a test build; build succeeds for Flatcar but fails for centos-7, amazon-2, and ubuntu-1804:

$ cd images/capi
$ CAPI_PROVIDER="ami" ./hack/image-build-flatcar.sh

[...]

==> Some builds didn't complete successfully and had errors:
--> centos-7: Error executing Ansible: Non-zero exit status: exit status 2
--> amazon-2: Error executing Ansible: Non-zero exit status: exit status 2
--> ubuntu-1804: Error executing Ansible: Non-zero exit status: exit status 2

==> Builds finished. The artifacts of successful builds are:
--> flatcar-stable: AMIs were created:
ap-northeast-1: ami-01422f07cb229aa95
ap-northeast-2: ami-0a4a3d31879fc4719
ap-south-1: ami-0605cb8a1d965d7fb
ap-southeast-1: ami-023fc401835929a6c
ap-southeast-2: ami-08a79c3a8bee57a2d
ca-central-1: ami-06a18c5d47031ef62
eu-central-1: ami-0002293abfd99a609
eu-west-1: ami-0ebbb3ce430f59629
eu-west-2: ami-0fd2b7086edf76724
eu-west-3: ami-0a078df8944a12627
sa-east-1: ami-066c05121be296ceb
us-east-1: ami-00e39a4d2d92c4648
us-east-2: ami-08c577590941697bd
us-west-1: ami-06ba65e4fc8ef60f5
us-west-2: ami-02c4354cb951df641

Is this expected?

[dongsupark]

I have not tested non-Flatcar OS with this PR.
Actually it is already a challenge to make it run for all non-Flatcar OSes. It is not actually a topic within this PR.

[t-lo]

Gotcha, thanks. All good then, will approve.

[dongsupark]

Merged to kubernetes-sigs@9737ee3