Fix a bug with st2 execution get command.
Fix a bug with st2 execution get command.

Previously, result and trigger_instance attributes were incorrectly excluded
from the API response if the user wanted to display a child property (e.g.
result.stdout) and the CLI always returned None for the attribute value.

pixelrebel/st2,StackStorm/st2,Plexxi/st2,peak6/st2,tonybaloney/st2,emedvedev/st2,StackStorm/st2,lakshmi-kannan/st2,peak6/st2,StackStorm/st2,nzlosh/st2,StackStorm/st2,lakshmi-kannan/st2,lakshmi-kannan/st2,nzlosh/st2,tonybaloney/st2,Plexxi/st2,emedvedev/st2,punalpatel/st2,nzlosh/st2,peak6/st2,tonybaloney/st2,emedvedev/st2,pixelrebel/st2,nzlosh/st2,punalpatel/st2,pixelrebel/st2,punalpatel/st2,Plexxi/st2,Plexxi/st2