added new play_hosts var

this variable has the 'current host list' to be executed over in the
play. Useful when using --limit to not iterate over hosts not included
in play in templates or with_items.

Signed-off-by: Brian Coca <briancoca+dev@gmail.com>
Merge pull request #5771 from bcoca/play_hosts2

added new play_hosts var
Updating changelog and docs for the new play_hosts variable
Make tmp paths in TestRunner dynamic based on pid

Fixes #4723