Improvements to the rax module

* Support state=absent with group or a group meta key
* Add auto_increment attribute to allow specification of a group without forcing an auto incrementing host number
* Add additional example that showcases exact_count functionality
* Don't use a file handle for files to upload to the server, read in the contents to avoid an issue where the file handle wasn't reset and an empty file was being uploaded
Merge pull request #5404 from sivel/rax-improvements

rax module: improvements
Update search for destination path
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Add keys() method to caching plugin.
Add plugin hooks for delete/contains.
Update code to handle fact caching with non-dict implementations