Add a metadata field for Nodes

[davidban77]

The idea of the meta field is to store values that can be useful for external applications like:

node01.meta.network_os = "ios"
node01.meta.platform = "xk9k"

The idea is that meta be user-driven, meaning that it can be modified/altered to the user needs and this field is not taken into consideration when interacting with GNS3 server.