[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 code could also be simplified so you can get rid of polling variable and just use break.

Something like:

```python
nodes = driver.list_nodes()

try:
    created_node = [n for n in nodes if n.id == subid][0]
except IndexError:
    pass
else:
  break