test suite updated to measure the time needed for stats collection

test suite updated to measure the time needed for stats collection

there is a need to plot the measured time which is needed for stats
to be collected in operational ds (inventory)

the delay between two triggers to inventory was decreased to 1s to
get a more precise time

monitoring change period was incresed to 450s. If timeout occures,
particular test case will fail and max time (450s) will be stored as
collection time

Change-Id: I0fe436055e1859f0d3f7db93100955e7815cf2e6
Signed-off-by: Peter Gubka <pgubka@cisco.com>