Initial commit to split includes into static imports/dynamic includes
This implements the new include/import syntax for Ansible 2.4:
* include_{tasks,role,variables} = dynamic
* import_{playbook,tasks,role} = static
The old bare `include` will be considered deprecated, as will any use of the `static: {yes|no}` option.
This also adds docs for import/include and reorganizing the "Playbook Reuse" section of the documentation.
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