Added scaling parameters to HDP plugin config

Added SCALE_EXISTING_NG_COUNT and SCALE_NEW_NG_COUNT parameters to HDP plugin
config. The default behavior remains as before, but an additional option
is added. Cluster scaling is parameterized in order to add an ability
to re-use cluster scaling test in performance/benchmark testing.

Change-Id: Iba1570f978e4f44046e64f1d38b2f498e7a0c421
Merge "Added scaling parameters to HDP plugin config"