Refactor RegistrationBase._update_state() to be more testable.

Move the create/delete node code into separate methods for easier
testing and write all those tests.
Issue 49: Don't recursively set ACL permissions on created paths.

This commit changes the ACLs used when recursively creating
paths in Zookeeper. If a path is nested (/foo/bar/baz/host:22),
we want /foo/bar/baz/host:22 and /foo/bar/baz to be created with
the ACL configured when the KazooServiceRegistry class was
instantiated. However, /foo/bar we want to have the default OPEN
ACL permissions so that other clients can write into the path
as necessary.
Add #49 to the changelog.
Trigger _create_node_path() from the _update_state() method.

By making _create_path() simply re-raise NoNodeErrors and having
_update_state() handle them, it makes the code slightly easier to read
and test.
Merge pull request #53 from Nextdoor/issue_49

Avoid creating ACLs on the higher level container paths for node registrations.