diskimage-create.sh: refactor $platform handling
diskimage-create.sh: refactor $platform handling

* use a safer way to get the distribution ID (instead of the Name) from
  /etc/os-release, and lowercase it
* use a case-based matching for detecting from /etc/system-release,
  assigning values compatible with the os-release IDs
* turn the if-elif-else chains for $platform into case switches

Change-Id: I6a4cfd091aa53f03cadc4c90248b7400ef15564a

openstack/sahara-image-elements,uladz/sahara-image-elements,bigfootproject/sahara-image-elements,crobby/sahara-image-elements,uladz/sahara-image-elements,openstack/sahara-image-elements,crobby/sahara-image-elements,bigfootproject/sahara-image-elements