refactor(*): remove all use of "self" for receiver names
Merge pull request #400 from jonboulle/job

refactor(*): remove all use of "self" for receiver names
refactor(registry, tests): make keyPrefix configurable

Previously all registry access was hardcoded to a specific prefix
(/_coreos.com/fleet). This makes the endpoint configurable through both
fleet and the client. Accordingly, the functional tests are updated to
take advantage of this, which means they should no longer conflict with
any instances of fleet which happen to be running in the environment
used for functional testing.