an inconsistency in SliverAction(); when operating on the entire slice
we do the whole thing in the background, returning (almost) immediately.
Which makes sense, we expect the caller to poll for status after.

But when operating on a subset of slivers (nodes), we do it
synchronously, which means the caller is left waiting until we get
through rebooting all the nodes. As David pointed out, when rebooting
nodes in the openstack profile, this can take a long time as the VMs are
torn down. This leaves the user looking at a spinner modal for a long
time, which is not a nice UI feature.

So I added a local option to do slivers in the background, and return
immediately. I am doing the for restart and reload at the moment since
that is primarily what we use from the Portal.

Note that this has to push out to all clusters.