cyclades: Fix IndexError
gtools: Fix AttributeError

Check 'dry_run' attribute after ensuring that it is a cluster operation,
since adding tags to networks or nodes does not have the dry run option.
cyclades: Handle VMs connected to unknown networks

Catch InvalidBackendIdError when reconciling instances that are
connected to networks that do not belong to Synnefo.
pithos: Change quota error handling

Pithos backend used to catch errors generated by the Astakosclient
and in those cases would return 413 (Request Entity too Large).
However, quota limit failures are not the only failures can occur.
Therefore, 413 responses are not always appropriate.
To fix this problem, let the respective methods fail with an
AstakosClientException which is handled by the common api decorator.