[jsk_tools] add get_ros_client_hostnames_func

[sktometometo]

Add get_ros_client_hostnames func to get hostnames where ROS nodes exists.

This is useful for checking if there are some people using the robot.

[http://fetch1075:11311][133.11.216.34] sktometometo@Leopard
~/ros/ws_jsk_ros_pkg/src/jsk_common/jsk_tools $ ifconfig
docker0: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 172.17.0.1  netmask 255.255.0.0  broadcast 172.17.255.255
        ether 02:42:37:27:4d:61  txqueuelen 0  (イーサネット)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

enp67s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 133.11.216.34  netmask 255.255.255.0  broadcast 133.11.216.255
        inet6 fe80::18e5:f24c:5fe7:ef20  prefixlen 64  scopeid 0x20<link>
        ether 7c:10:c9:46:7f:3d  txqueuelen 1000  (イーサネット)
        RX packets 117620731  bytes 136866592196 (136.8 GB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 112791454  bytes 123286758329 (123.2 GB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device memory 0xb8900000-b891ffff  

enx70886b8f1af3: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        ether 70:88:6b:8f:1a:f3  txqueuelen 1000  (イーサネット)
        RX packets 5237200  bytes 4475983207 (4.4 GB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 1881910  bytes 127235839 (127.2 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (ローカルループバック)
        RX packets 522520  bytes 371160786 (371.1 MB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 522520  bytes 371160786 (371.1 MB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

[http://fetch1075:11311][133.11.216.34] sktometometo@Leopard
~/ros/ws_jsk_ros_pkg/src/jsk_common/jsk_tools $ get_ros_client_hostnames 
ERROR: Communication with node[http://133.11.216.168:45357/] failed!
133.11.216.168
133.11.216.36
fetch1075

[knorth55]

FYI: #1699