Make Bootstrap Return Endpoint Addresses

Make Environ.Bootstrap return State network addresses. This is
needed for a followup PR which will write those addresses to environ
config at the end of bootstrap. 

While Bootstrap currently should only return one address, that of machine 0, it is returned in a slice ([]network.Address) for future flexibility.

## Functional changes:
- Change Environ interface so Bootstrap returns State nework addresses:
    * environs/interface.go:105
 
- Return BootstrapArgs.Host for the manual provider
    * manual/bootstrap.go:126 (test: manual/bootstrap_test.go:82)

- Return "localhost" for the dummy and local providers:
    * provider/dummy/environs.go:698 (test: provider/dummy/environs.go:107)
    * provider/local/environ.go:204 (test: provider/local/environ.go:180)

- All other providers use common.Bootstrap. Return instance addresses for common.Bootstrap:
    * provider/common/bootstrap.go:88 (test: provider/common/bootstrap.go:181)

The rest of the PR updates functions that implement Environ.Bootstrap