Add workaround to have a static IP for containers

  - In some cases, like in a workshop where many people are running OVPL
    simultaneously, trying to find IP for newly created containers dynamically
by pinging results in IP conflicts among multiple containers. So the workaround
in these situtations is to have a static IP per running OVPL instance. So
basically a user using OVPL on their machine can deploy only one lab with one
container using a static IP mentioned in the config.json file. If this field is
left blank in the config file, OVPL behaves normally(find free IPs dynamically
in a subnet by pinging them).