add support for writing files to containers from cloud-init config

[europaul]

We are extending the support for the cloud-init config for containers. While the old config format with

ENV1=value1
ENV2=value2

still remains available we also introduce the support for the original cloud-init config syntax with support for fields runcmd (only for defining envs) and write_files, like in the following example

#cloud-config
runcmd:
  - ENV1=value1
  - ENV2=value2

write_files:
 - path: /etc/injected_file.txt
   owner: root:root
   permissions: '0644'
   encoding: b64
   content: YmxhYmxh

The section runcmd will only be used for providing envs, while the new section write_files can be used to write files to the containers in the usual cloud-init way. The option to change the owner is however not supported at the moment.

The rest of the field of the cloud-init configuration will be ignored.

[eriknordmark]

We do need to support the old format since the users have existing cloud-init configured for their edge applications.
So need to detect which format is in use to handle both.

[europaul]

brought back the support for old cloud-init format and fixed unit tests.
I also intend to add integration tests in eden to test the new functionality

[codecov]

Codecov Report

Attention: 107 lines in your changes are missing coverage. Please review.

Comparison is base (ccbcc7d) 20.51% compared to head (e71e15e) 20.53%.
Report is 35 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3520      +/-   ##
==========================================
+ Coverage   20.51%   20.53%   +0.02%     
==========================================
  Files         208      209       +1     
  Lines       45381    45512     +131     
==========================================
+ Hits         9308     9345      +37     
- Misses      35390    35479      +89     
- Partials      683      688       +5     

Files	Coverage Δ
pkg/pillar/types/domainmgrtypes.go	0.00% <ø> (ø)
pkg/pillar/utils/cloudconfig/cloudconfig.go	57.62% <57.62%> (ø)
pkg/pillar/cmd/domainmgr/domainmgr.go	0.40% <2.38%> (-0.05%)	⬇️

... and 1 file with indirect coverage changes

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[eriknordmark]

You can ignore those yetus complaints. It is supposed to only look at lines/functions which were changed in your PR but that doesn't always work.

[eriknordmark]

Still some minor issues, but approving so we can get some test results.

[europaul]

added tests for the new cloudconfig package

[eriknordmark]

Can you update the PR description to capture that we do handle the old format.

Also, is there any file in docs or pkg/pillar/docs which specifies the env=value format? Any one discuss cloud-init or userdata where we should add such specification?
It might make sense to rewrite/expand the first paragraph in docs/ECO-METADATA.md for this.

Feel free to merge this and do that doc update in a separate PR if you'd like.

[europaul]

@eriknordmark I updated the PR description, but I don't have the write access to the repo to merge.

I think we should create a new doc file docs/CLOUD-INIT.md where we describe how our ECOs (VMs and containers) support cloud-init configuration.

[eriknordmark]

I think we should create a new doc file docs/CLOUD-INIT.md where we describe how our ECOs (VMs and containers) support cloud-init configuration.

Makes sense. I'll merge this for now