add bracket on node if using proxy and node has ipv6 address (#2245)

* add brackets "[]" if @node.ip is ipv6 using proxy

* add ipaddress dependency

* remove ipaddress gem dependency

* remove ipaddress gem dependency

opted to use "include" to validate if the ip address has the colon `:` string, instead of `ipaddress` gem

* removing `if` statement

looks like `[ip]` works even with ipv4 address.
so no need for an if statement.
much simpler change

lib/oxidized/input/ssh.rb

@@ -140,7 +140,7 @@ def make_ssh_opts
         if (proxy_port = vars(:ssh_proxy_port))
           proxy_command += "-p #{proxy_port} "
         end
-        proxy_command += "#{proxy_host} -W %h:%p"
+        proxy_command += "#{proxy_host} -W [%h]:%p"
         proxy = Net::SSH::Proxy::Command.new(proxy_command)
         ssh_opts[:proxy] = proxy
       end