Allow to use an arbitrary numeric identifier for NamedInstance

Allow to use an arbitrary numeric identifier for NamedInstance

In some cases, the numeric identifier doesn't correspond to the
directory name (i.e. `resolver` server in `shutdown` test, which is
supposed to be 10.53.0.3). These are typically servers that shouldn't be
auto-started by the runner, thus avoiding the typical `*ns<X>` name.

Support these server by allowing a fallback initialization with custom
numeric identifier in case it can't be parsed from the directory name.