release_tool: Fix bug when assigning branch for a beta build.
release_tool: Fix bug when assigning branch for a beta build.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@mender.io>

pasinskim/integration,pasinskim/integration,GregorioDiStefano/integration,pasinskim/integration,GregorioDiStefano/integration