cyclades: Validate VM metadata on create/update

* Check for VM max metadata items when updating/creating a single item.
* Check the length of metadata items when updating metadata items.
astakos: Fix a unicode bug in user-add

Fix a unicode bug in snf-manage user-add when the username contained
non-ascii characters.
astakos: Add checks for quota limits

Check that total (project) quota limit is no less than member limit;
and that both limits are not negative.