This uses Flakka (a custom Akka 2.3 build) to resolve the issue that
the bind address needs to be matching the external address of the
JobManager. With the changes applied, we can now bind to all
interfaces, e.g. via 0.0.0.0 (IPv4) or :: (IPv6).

For this to work properly, the configuration entry
JOB_MANAGER_IPC_ADDRESS now represents the external address of the
JobManager. Consequently, it should not be resolved to an IP address
anymore because it may not be resolvable from within containered
environments. Akka treats this address as the logical address. Any
messages which are not tagged with this address will be received by
the Actor System (because we listen on all interfaces) but will be
dropped subsequently. In addition, we need the external address for
the JobManager to be able to publish it to Zookeeper for HA setups.

Flakka: https://github.com/mxm/flakka
Patch applied: https://github.com/akka/akka/pull/15610

- convert host to lower case
- use consistent format for IPv6 address
- adapt config and test cases
- adapt documentation to clarify the address config entry
- TaskManager: resolve the initial hostname of the StandaloneLeaderRetrievalService

This closes #2917.