JSON for virtual cluster

[laszewsk]

a) rename virtual-cluster to virtual-cluster.cfg

b) introduce logic (if not already done) that the file is taken as follows

1. check first if it is located in the cwd
2. if it is not there check the location from ~/.futuregrid/virtual-cluster.json
3. if its not there use the location specified in futuregrid.cfg, wherever that is located base on or other logic.

c) change the object formt in which you store the object to json

d) discuss this with Gregor before implementation to identify advantage/disadvantage of the current approach with this method.

e) if we find that your current mehd is better we keep that and close this issue.

I think it may be nice tp be able to read the file more easily, but if we have commands like fg-cluster list/history that may not be needed.