fate: support testing of release branches

Adding 'branch=release/10' to the fate config file will check the
release/10 branch instead of master. If no branch is specified it will
use 'master' so that existing config are still valid.

The server side changes are already deployed, see
https://fate.libav.org/v10/ for an example. The server supports only the
release/* branches.

The server enforces that a single slot tests always the same branch.
Please append "-v$RELEASE" to the slot of release branch configs or make
the slot otherwise unique.

A different fate samples dir is needed for each release branch. make
fate-rsync has the correct URL in each branch.