Allow displaying only the last revision of bundles in bundles-containing.

Update the bundles-containing endpoint to introduce the only-latest flag.
When activated, only the last revision of each bundle containing the given charm is returned.
This branch fixes #215 

I'm not entirely sure if it would be better or not, but you could phrase this the other way round.
```
if charmId.name != id.Name {
    continue
}
...
return true
```