Default `keep_associated_vpc` to `true`

[dentarg]

If keep_associated_vpc is set to false (the default), when an instance is deleted, if the associated VPC is empty, the VPC will also be removed. For Terraform managed resources, it makes sense to never implicitly remove resources (see #238 for example).

In the next major version of the provider, we will change keep_associated_vpc to default to true.