Refactor meta server related logic so that apollo-core is common enough to go maven central repository.

1. Remove apollo-env.properties from apollo-core, so that it's possible
to deploy apollo-client to maven central repository.
2. Add MetaServerProvider spi so that users could customize how they
find apollo meta servers.
3. Refactor env related utils so that Env.UNKNOWN is returned for
invalid env string instead of null.