Fix bugs in build_ami.yaml
Fix bugs in build_ami.yaml

- This sets a default value for instance_tag in case it is defined but
  set to a empty value.
- Fixes #268

vikaslaad/svt,vikaslaad/svt,mrsiano/svt,vikaslaad/svt,mrsiano/svt,mrsiano/svt,mrsiano/svt,vikaslaad/svt,vikaslaad/svt,mffiedler/svt,mffiedler/svt,mrsiano/svt,mffiedler/svt