health-check: Reporting needed time for storage bench

When running the storage bench, it's nice to know how long it will take.
Let's compute the needed time and display it.

A typical output will be:

Running storage bench in non destructive mode for 90 seconds
Benchmarking storage sda for 10 seconds in randread mode with blocksize=4k
Benchmarking storage sda for 10 seconds in read mode with blocksize=1M
Benchmarking storage sdb for 10 seconds in randread mode with blocksize=4k
Benchmarking storage sdb for 10 seconds in read mode with blocksize=1M
Benchmarking storage sda,sdb for 10 seconds in randread mode with blocksize=4k
Benchmarking storage sda,sdb for 10 seconds in read mode with blocksize=1M