qa/workunits/rgw: drop netstat usage

qa/workunits/rgw: drop netstat usage

The `netstat` is deprecated now in modern Linux and usually
requires an extra package dependency to be installed.
Usually it is `net-tools`, however, for example, opensuse,
`netstat` does not present in it. Thus, let us use `ss` as
an alternative.

When using `netstat -nltp` we get lines like:
   'tcp        0      0 0.0.0.0:443             0.0.0.0:*               LISTEN      25156/valgrind.bin  \ntcp6       0      0 :::443                  :::*                    LISTEN      25156/valgrind.bin  \n'
When using `ss -nltp` we get lines like:
   'LISTEN 0      4096           0.0.0.0:443       0.0.0.0:*    users:(("memcheck-amd64-",pid=66045,fd=72))'
so we need to filter processes by `memcheck`. However further
parsing code works equivalently as for netstat.

Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@clyso.com>