Allow 0 value in vcd_nsxt_alb_edgegateway_service_engine_group.reserved_virtual_services field

[Didainius]

Closes #923

Resource vcd_nsxt_alb_edgegateway_service_engine_group field reserved_virtual_services hit a problem where it cannot differentiate between 0 and empty value. The reason for it is that Terraform SDK has a limitation for schema.TypeInt value. (more about it).

The way to overcome it is to use schema.TypeString instead of schema.TypeInt. This works because Terraform does not put explicit differences between int and string in HCL. In the code we convert Int <-> string to match this field. Users would not even notice this change directly, because Terraform has weak typing and you can supply int value to a TypeString field (wihout quotes).

To avoid setting some arbitrary string value, there is a schema validator ValidateFunc: IsIntAndAtLeast(0).

Testing acceptance, binary and upgrade tests with tag alb passed

[Didainius]

Confirmation it works for user - #923 (comment)

[adambarreiro]

Approved! I think it's a correct workaround. Recently I faced a similar nuance with TypeBool, as nil and false are indistinguishable when reading unset Computed attributes.

[lvirbalas]

Nice fix!