hazelcast.group.name/password in distributions is not a placeholder

When the Stash distribution is built, Maven filtering replaces

hazelcast.group.name=${user.name}
hazelcast.group.password=${user.name}

with

hazelcast.group.name=bamboo-agent
hazelcast.group.password=bamboo-agent

That user.name is intended to be resolved at runtime, not at build time. Having it embedded with a set value when the distribution is built makes Stash instances more willing to cluster with each other by default than they should be. This is particularly problematic for plugin developers who run Stash using AMPS, because the default data used by AMPS has a clustered license and, since Stash 3.6.0, multicasting is enabled by default.