Fix ID field casing convention in client.go

[displague]

ID/UUID appears to be the convention we've been using in ne-go. We have many examples of ...UUID, ...ID, IP..., and MAC....

We have some public type definitions that don't fit the current convention.

$ git grep -n  -e Id -e Uuid client.go rest_client.go
client.go:249:  MgmtAclTemplateUuid *string
client.go:263:  WanInterfaceId      *string
client.go:435:  ClusterId          *string
client.go:446:  LicenseFileId       *string

This issue is a continuation of #13 where the internal representation of these types was updated.

Addressing this issue may constitute a breaking change so it was deferred from #13.

Originally posted by @displague in #12 (comment)