[booth] `pcs booth sync` does not synchronize configuration to other clusters

This especially makes that command useless when degenerate
single node clusters are considered, but nonetheless, the point of
the management tool is to make the typical workflows easy, and
"sync across the clusters" is one such typical use case for multisite
nonetheless -- that's exactly what you want to do with `booth sync`,
don't you?!

Relevant issue here: #224
Note that the motivation example there already works with the
mechanism of "virtual (booth) IP to cluster mapping", which is
exactly what fits into this picture. Good news is that `pcsd` listening
on `[::]:*` by default and, as tested, will gladly serve the requests
coming via a virtual IP introduced even once `pcsd` got started.