Do not break the VM list when a VM's OpenNebula template was deleted

The "my virtual machines" page showed "We could not load your VMs due to a
backend connection error" for any user owning a VM whose template had been
removed from OpenNebula. OpenNebulaManager.get_template() converted every
failure, including WrongIdError for a missing template, into
ConnectionRefusedError, and VirtualMachineSerializer.get_configuration()
let that propagate, so one VM with a stale template id took down the whole
list. The only log output was an empty ERROR line, because WrongIdError
has no message.

* Add opennebula_api.exceptions.TemplateNotFoundError and raise it from
  get_template() for a missing template; keep ConnectionRefusedError for
  genuine backend failures and log them with a traceback.
* Add OpenNebulaManager.get_template_names() and resolve_template_id() so
  the serializer fetches the template pool once per list instead of
  constructing an OpenNebulaManager and fetching the pool for every VM.
* get_configuration() now falls back to the template name embedded in the
  VM name ("<template>-<vm id>") when the template is missing or the pool
  cannot be fetched, and logs why.
* Replace str.lstrip('public-') with a real prefix strip (lstrip removes a
  character set, so "public-ubuntu" became "ntu") and also strip
  "ipv6only-".
* ProcessVMSelectionMixin shows "configuration is no longer available"
  instead of an unhandled error when ordering a retired template.
* VirtualMachinesPlanListView logs the exception behind the banner.
* Add mock-based unit tests for the fallback, the UPDATED_TEMPLATES
  mapping, the single pool fetch and the new exception.

The UPDATED_TEMPLATES setting keeps working as an operational override for
mapping retired template ids to their replacements.

opennebula_api/exceptions.py

@@ -8,3 +8,13 @@ class UserExistsError(Exception):
 
 class UserCredentialError(Exception):
     pass
+
+
+class TemplateNotFoundError(Exception):
+    """Raised when an OpenNebula VM template id does not exist (any more)."""
+
+    def __init__(self, template_id):
+        super(TemplateNotFoundError, self).__init__(
+            "OpenNebula template %s does not exist" % template_id
+        )
+        self.template_id = template_id