Fix histogram ylimit for multiple executions (#1661)
Fix histogram ylimit for multiple executions (#1661)

In the plot_histogram function there was a bug when there were multiple
executions passed into the function with the y axes bounds. We were
previously setting the ylimit for each iteration of loop which goes over
each execution. However we should look at the max probability across all
executions when determining the upper limit for the yaxis. This commit
fixes this change by saving all the probabilities in a list outside the
loop and then moving the ax.ylim() to the end of that loop.

Fixes: #1660
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py