Multiple stage servers in open build service

 The open build service publisher has a configuration variable in BSConfig.pm, where you can define a rsync server to publish the built repos to.

Unfortunately, the documentation apart from the actual code (in src/backend/bs_publish function sync_to_stage) seems scarce, so let's document one non-standard usage here.

"Standard" (IMO) usage, one rsync staging server:

our $stageserver = 'rsync://my-rsync/obs-repos-module';

 But now, for a transition phase, I need multiple rsync servers that are all synced to. The format for this is a perl array variable reference that contains pairs of "project name regex, array of sync servers". This also allows to sync different repositories to different rsync servers for example.

The simplest use of this is "sync all repos to multiple rsync servers" and is configured like this:

our $stageserver = ['.*', ['rsync://rsync1/module1', 'rsync://rsync2/module2', 'rsync://rsync3/module3']];

 With this configuration, bs_publish will send all repos to the three mentioned rsync URLs in turn.