release_tool: Fix --pr argument not working after adding dynamic params.
release_tool: Fix --pr argument not working after adding dynamic params.

We were referring to the REPOS keys directly, which are container
names, but we need to use Git names, and then we have no choice but to
traverse the whole list.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@northern.tech>

pasinskim/integration,pasinskim/integration,pasinskim/integration