tests: use environment instead of host

[jdavid]

Comes from #120

Suggested pseudo code by paddybyers

environment = os.environ.get('ABLY_ENV', 'sandbox')
host = (environment == 'production) ? 'rest.ably.io' : environment + '_rest.ably.io'
port = (environment == 'local') ? 8080 : 80
tls_port = (environment == 'local') ? 8081 : 443

In test/ably/restsetup.py

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