added env uuid to the state object

Add the environment uuid to the state so that we can pass it on to AddEnvUser in a future pr
```
c.Assert(st.EnvironTag(), gc.Equals, names.NewEnvironTag(env.UUID()
entity, err := s.State.FineEntity(st.EnvironTag().String()
```
or
```
c.Assert(st.EnvironTag().Id(), gc.Equals, env.UUID())
entity, err := s.State.FineEntity(st.EnvironTag().String()
```