Adding new playbook objects for v2

* Playbook
* TaskInclude
Fix path to the integration tests

"tests/integration" -> "test/integration"
Merge pull request #9500 from alexandershov/devel

Fix path to the integration tests in docsite
add documentation for the delegate_to magic variable
Merge pull request #9502 from nikut/delegate_docfix

add documentation for the delegate_to magic variable
changed examples to not have a  non working variable that gets confused
with directives as per #9264
Merge pull request #9503 from bcoca/doc_fix_uservar

changed examples to not have a  non working variable
added error handling on private key stat in case it is saved in agent
but not in filesystem
Merge pull request #9509 from bcoca/no_ssh_fail_missing_keyfile

added error handling on private key stat in case it is saved in agent