Bootstrap no longer cleans up if it can't tell whether an environment is bootstrapped or not.

Added a test to simulate an instance in which this might occur. Tested locally; some tests were failing, but they looked innocuous and related to connecting to mongo, not these changes.
Related to https://bugs.launchpad.net/juju-core/+bug/1340893
As long as it doesn't make things unreadable, I consider it good practice to combine the assignment and condition so that it's scoped. This means I don't have to think about whether that variable is used, affects shadowing, etc., outside of that scope.

i..e.

```go
if environInfo, err := store.ReadInfo(envName); err == nil {
   ...
}
```