Ready service

Previous discussions, incl. a session at last year's ODL DDF, have repeatedly identified the need to have a general way to reliably detect when ODL is "ready".

Today projects sometimes have ad-hoc solution for this; for example netvirt's org.opendaylight.netvirt.statemanager.StateManager (which logs "StateManager all is ready") is related to (the lack of a general solution for) this.

I've hit what's basically the same gap again while contributing an "auto-import-on-boot" feature to the Daexim project in https://git.opendaylight.org/gerrit/#/c/55035/, where the Daexim committers would like that feature to hold importing until "the system is full ready" (I don't fully understand why, but that is not the point of this bug), and will therefore as part of the work I'm doing in that context contribute a general new Ready Service, to infrautils.

This will build on top of the work I've done for the Extended SingleFeatureTest (SFT) incl. TestBundleDiag in odlparent (see e.g. https://lists.opendaylight.org/pipermail/release/2017-January/009062.html), and actually re-use that same code.

The first contribution for this will be minimalistic and fulfil the immediate need I have (in Daexim). Later Enhancements by others or myself on top of that first iteration can then obviously extend it; ideas for possible future follow-up improvements which won't be in my initial proposal include but are not limited to:

1. YANG model to expose the simple API I'll offer as RESTCONF etc. RPC

2. YANG data model which sets a flag in the data store (like netvirt's StateManager)

3. JMX ?

3. ...