Add POSIX compatibility

[jkroepke]

Hi,

it's almost a rewrite of the shell script for helm script. I hope the new maintainer / well-known company @zendesk will have some time for this project.

I remove all bash related shell code and replace it with POSIX feature. The script will run on almost any posix compatible shell. The scripts is also compliant to shellcheck.

ZSH (MacOS default shell) and ash (Alpine default shell) should now out of the box supported. gnu-getops is not required anymore.

All tests from test.sh are green under MacOS 10.15.4 (zsh, bash 3.2.57) and Alpine 3.11 (ash)

Additional features:

All helm commands are supported now. Before this only install|template|upgrade|lint|diff are supported. If plugins like kubeval are installed this script need to be modified. The PR remove the limitation.

I recommend split view and ignore whitespaces for best experience to get all changes. https://github.com/zendesk/helm-secrets/pull/153/files?diff=split&w=1

Includes/Resolves/Fixes:

• https://github.com/zendesk/helm-secrets/pull/149
• https://github.com/zendesk/helm-secrets/pull/150
• https://github.com/zendesk/helm-secrets/pull/129 (All name restrictions dropped)
• https://github.com/zendesk/helm-secrets/pull/143
• https://github.com/zendesk/helm-secrets/pull/142 (dpkg variant dropped; you should never run helm plugin install as root)
• https://github.com/zendesk/helm-secrets/pull/136 (verbose output is not on stderr)

[jkroepke]

I'm gonna start to maintain an own fork. https://github.com/jkroepke/helm-secrets

If you still interest into this PR, contact me.