Some attributes needed to be changed in the test xmls. With this
commit all the description files are consistent with the new co-
occurrence constraints.
Now each user item may or may not have a groups attribute. The
groups attribute is a comma separated list of group names. If
present the first group name will be the login or primary group,
if not present, the underlying toolchain will assign the default
login group for that user.
An instance of RuntimeChecker allows to semantically check for
error conditions according to the provided image description
and build options. Related to #20
