Small improvements for release script
Small improvements for release script

- Print usage if no tree-ish (tag, branch or commit) was
  specified.
- Use “gntrelease.XXXXXXXXXX” as temporary directory
  template. This makes it easier to find all of them.
- Compute MD5 checksum. This doesn't cost a lot and
  might come in handy one day.

Signed-off-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>

bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,leshchevds/ganeti,dimara/ganeti,grnet/snf-ganeti,andir/ganeti,yiannist/ganeti,onponomarev/ganeti,leshchevds/ganeti,dimara/ganeti,grnet/snf-ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,ganeti/ganeti,apyrgio/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,ganeti/ganeti,mbakke/ganeti,mbakke/ganeti,leshchevds/ganeti,mbakke/ganeti,ganeti/ganeti,yiannist/ganeti,apyrgio/snf-ganeti,andir/ganeti,andir/ganeti