* vmware_inventory: permit to group by custom field
This permits to create instances, affect some custom fields like EC2 tags and then retrieve groups from custom fields like EC2 inventory
* vmware_inventory: Customize skip_keys & add resourceconfig to skip_keys
Verify if customfield is a str before processing custom fields for a host
* Fix many points reported by PyCharm as PEP 8 code style
* Improve inventory performance by dropping vim.HostSystem & vim.VirtualMachine collect when depth >= 2
* Declare some class variables properly
* Remove some unused variables
* Add documentation in vmware_inventory.ini for VMWARE_USERNAME & VMWARE_PASSWORD env vars
* vmware_inventory: fix the --host option
* Fix skip_key evaluation
* Short circuit deep dives in datastores and resourcegroups
* Put timestamps in the debug output and add a few more
* Implement a user defined proplist to increase performance
* Make all props into dicts
* Update ini with example
* Fix tests
cache_path is used to calculate cache_dir , the script doesn't actually read cache_dir from this file.
This makes the setting work (otherwise it always uses the default).
