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
Add independent disk capabilities

Independent disks are available to use only through the api in 5.1, not
the console.  Added methods to create, query, attach to a vm, detach,
and delete.
Update CHANGELOG
Return also network ID in get_vapp
rasd:Parent in add_disk could be different than '1'

For now we try to reuse an existing value and raise an error when:
- a valid disk doesn't exist
- different disks exist with different rasd:Parent

The official documentation doesn't really help here:
http://pubs.vmware.com/vcd-55/index.jsp#com.vmware.vcloud.api.doc_55/GUID-E56EEEEA-EDED-4EBC-8095-08AD2668F0D2.html
Check .ovf extension in upload_ovf

No check was present to avoid uploading a file different from ovf.
Use VCR to make tests more readable/manageable