`berks install` should not write a locked version for a cookbook installed by `metadata`

When `berk install` is run from within a cookbook and the Berksfile contains a `metadata` key, the Lockfile will have a constraint set for the current working cookbook.

This is a problem when the version number is changed and another install operation occurs. See below:

```
$ berks install
$ berks upload
The cookbook downloaded from path: '...'
  something (0.2.16)
does not satisfy the version constraint:
  something (= 0.2.15)
```

It is expected that the constraint would either be ignored, or not set, for the current cookbook