CI: Harmonize exec_script() API

Instead of making the function a class member, make it an instance
member because (1) it is more aligned with other functions, notably,
run(), and (2) especially for RemoteCmd() it can be faster, as
successive exec_script() calls do not require a new SSH connection.

ci-scripts/cls_corenetwork.py

@@ -77,7 +77,8 @@ class CoreNetwork:
 		words = line[1:].strip().split(" ")
 		script_name = words[0]
 		options = " ".join(words[1:])
-		ret = cls_cmd.runScript(host, script_name, 300, parameters=options, silent=silent)
+		with cls_cmd.getConnection(host) as c:
+			ret = c.exec_script(script_name, 300, parameters=options, silent=silent)
 		return ret
 
 	def _command(self, cmd_list, must_succeed=False, silent=False):