Now 'fuel2 graph download' command can be executed without
specifying any task level parameters that leads to erroneous
behavior, i.e. plugins task level graph is downloading by default.
This patch fixes such behavior and makes task level parameters
mandatory, e.g.:
fuel2 graph download [-h] -e ENV (-a | -c | -p | -r)
[-t GRAPH_TYPE] [-f FILE]
Change-Id: I5ed561cda9b0d35043822d01b6bb0b37df451fcb