Create List functions for entities that don't have one

[dataclouder]

As support for test refactoring, we need the ability of listing several entities.
Currently we are missing public functions for:

• org
• edgegateway
• edgegateway_vpn
• dnat
• snat
• firewall_rules
• nsxv_snat
• vapp_network
• independent disk
• media items
• ejectable media

We can get the list of other entities by querying the parent entity. For some of these entities we have private functions that can be converted to public ones.

The main purpose of this issue is the have the ability of filling the test configuration files for both g-vcloud-director and terraform-provider-vcd with minimal input from users. i.e. by detecting whether the needed elements are available.

The secondary purpose is to clean up the environment when tests fail in unpredictable way, leaving the environment unsuitable for another test run.