default "" as var.vpc_ip_range (optional in digitalocean provider)

Description

Provides default empty value for the vpc_ip_range variable, since it is an optional argument for digitalocean_vpc.

Supporting information

#80 (closed)

Testing instructions

Steps to test the changes:

1. TBD

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated