Support of ubuntu-core was dropped and now it is impossible to
continue using it. Therefore, start using ubuntu-minimal OS element
for building image.
Also, add 'other-requirements.txt' file with list of required system
packages to be installed prior to buidling image.
Change-Id: Ief7059b9e6cc2aad03c81225663f5313a8d2f5f2
Closes-Bug: #1586351