added operation get_by_name for OrgVdcNetwork

If you are wondering, where are tests for this. ;-), they are in
vcloud-tools. Currently most of the integration tests lie in vcloud-tools. I
have added a test around this in vcloud-tools repo. 

we need to discuss how do we want to write tests for vcloud-core 
- either it can be tested along with vcloud-tools. 
- or we maintain tests in both repos. this can be painful.

@annashipman : pls take a look.