api: Preserve environment variables when calling a command

[matejmatuska]

When calling a command with _call and env parameter is not None, create and modify a copy of caller process environment variables and call the command with the copy in order to preserve them.

Jira ref: OAMG-7203

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[centos-ci]

Can one of the admins verify this patch?

[leapp-bot]

This PR has been linked in issue tracker (#OAMG-7209).

[vinzenz]

Very nice, thanks a lot!