Fix state whitelist in rack update

Whitelist shouldn't contain 'provisioned', because it's not a valid
state. It should contain 'active' instead [1].

[1] https://github.com/tuskar/tuskar/blob/40aee8f65bfa4a477149895c92a3f02e23baf1dc/tuskar/db/sqlalchemy/api.py#L289
Merge pull request #81 from jistr/bug/rack_update_state_whitelist

Fix state whitelist in rack update