Change EnsureNotBootstrapped to use StateServerInstances

EnsureNotBootstrapped is modified to call StateServerInstances,
rather than LoadState directly. StateServerInstances is now
required to return ErrNotBootstrapped if there are no state
server instances.

The BootstrapState type and related functions have been moved
from environs/bootstrap into provider/common, which is where
it should have been before were it not for the layer violation.
We need a test for this method, is there one already? If so, it would need updating to check the new behaviour