trigger/compete with :VARIABLE syntax ?

https://jira.ecmwf.int/browse/PYFLOW-23

we can refer to variable with absolute path
yet in native API we can use relative path for variable in triggers / complete expression like 
complete :SUITE_START ne :YMD
which is appreciated while it has benefit absolute path does not provide

it seems this is not natively acceptable syntax with pyflow at the moment

