Add pgbench init and pgbench command for node.

[stalkerg]

Pgbench run as Popen for background tests. Example usage:

node.pgbench_init()
pgbench = node.pgbench(
    stdout=subprocess.PIPE,
    stderr=subprocess.STDOUT,
    options=["-c", "4"])
pgbench.wait()
pgbench.stdout.close()