models: Return string from 'combined_check_status'

models: Return string from 'combined_check_status'

Previously the 'combined_check_status' function returned the checks
enum integer for the status field. However, the string representation
is more meaningful and is the only representation seen by users at
moment. Change this function so it returns, for example, 'success'
instead of '1'.

Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Andy Doan <andy.doan@linaro.org>