[New Feature] add supporting vultr compute driver

vultr compute driver for libcloud.
added testcases for vultr compute driver
added below commands

list_nodes
list_locations
list_images
list_sizes
reboot_node
destroy_node

This function seems to assume that API always returns a list.

If it's only used in a context where API returns a list (e.g. list methods), then it should probably have a better name (e.g. `_list_resources` or something) and this should be documented in the docstring.

The method should also be "private" (underscore prefix).