Cache the results for the RackSpace dynamic inventory.

This patch to the RackSpace dynamic inventory plugin makes it cache results for 10m (similar to the EC2 plugin). Running the complete API call for large sites every time can be very slow. Like the EC2 inventory you can force-refresh the cache by running with --refresh-cache, or by deleting the cache file from the temp dir.

@angstwad @j2sol @sivel

The cache path used by DO and ec2 is configurable.  I am in the process of adding a config file for `rax.py`.  The default in `ec2.ini` is `~/.ansible/tmp` in `digital_ocean.ini` it is `/tmp`.  I'm still not a huge fan of the default being `/tmp`

Perhaps for now we can just hard code it to  `~/.ansible/tmp` and make it configurable later.