The lock file is (a temporary file) opened in the parent process, whose
open fd is inherited by the workers after fork, and passed down through
the PlayContext. Connection grows lock/unlock methods which can be used
by individual connection plugins.
* Prompt had to be moved up, as it needed to be done before the first
templating of the play occurs, otherwise vars_files won't be templated
properly
* Fixed a bug related to an earlier fix of vars_files incorporating
extra vars to do the templating of the file name
Fixes#11404
Also making PlayContext a child class of the Playbook Base class,
which gives it access to all of the FieldAttribute code to ensure
field values are correctly typed after post_validation
Fixes#11381
