Fix add network to VM for API v.5.5

/vApp/vm-#{vmId}/networkConnectionSection API does not contain
network "none" anymore.
It has to be removed only if present.
This change is not documented in official doc nor release notes.
Add "media" item type management to get_catalog_item
Add API call to discard suspended state of a vApp or VM.

Official documentation:
http://pubs.vmware.com/vcd-55/topic/com.vmware.vcloud.api.reference.doc_55/doc/operations/POST-DiscardVAppState.html
Fix VM IP address retrieval with different interfaces on same network

Now get_vm appends NetworkConnectionIndex to network name to generate a
unique hash key
Update CHANGELOG
Use VCR to make tests more readable/manageable