Support more provisioning options via 'facility' environment variable

[heyste]

When provisioning a new pair instance it would be helpful to use the environment variable facility to either;

• deploy servers to locations based on the most available capacity for the requested server type (facility=any)
• deploy servers to only locations based on the most available capacity for the requested server type at those locations set in an array. ( facility=['dc13','ny5'] )

Reference: https://metal.equinix.com/developers/docs/deploy/batch-deployment/#specifying-facilities