Don't capture STDERR in run_cmd by default
Don't capture STDERR in run_cmd by default

Setting stderr to subprocess.STDOUT means it's capture whenever
STDOUT is being captured. Setting it to None by default makes run_cmd
only capture STDOUT. Can be overwritten by passing `subprocess.STDOUT`
as argument value for `stdout`.

We use output capture to parse results of git commands, but they're
written to STDOUT so this change shouldn't affect the release process.

alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws,alphagov/digitalmarketplace-aws