Fix name of branch_name variable

[andrewmogan]

Responding to issue #362, looks like I just forgot to name branch to branch_name. I've tested locally that supplying an overwrite_branch to DAQRelease works as expected, e.g., I can checkout production/v4 instead of develop and generate the repos correctly.

Also, I realized there's no obvious reason to set a default value for branch_name, so it's now a required argument.

[jcfreeman2]

Looks good.