Adding new Keyword "Execute in Container", similar to `kubectl exec`

[Nilsty]

New Keyword "Execute in Container"

"Execute in Container" will allow the execution of shell commands in a container similar to kubectl exec

Fixes #81 The command kubectl cp is using the same functionality as kubectl exec under the hood. I added a few test case example which use this keyword to copy a file from a container.

Before merge following needs to be applied:

• [x] At least one example testcase added in testcases/
• [x] Library Documentation regenerated according to Generate docs
• [ ] PR entry added in CHANGELOG.md in In progress section
• [x] All new testcases tagged as prerelease along other tags to exclude it from execution until released on PyPI
• [ ] Coverage threshold increased in .coveragerc if new coverage is higher than actual, see the lint-and-coverage step in CI

fail_under = 67

[MarcinMaciaszek]

@Nilsty is there any chance to fix it shortly? I would like to use exec :)

[MarcinMaciaszek]

@m-wcislo this PR can be removed ;-)