ipam-client improvements

 ipam-client improvements (#32)

* ipam-client: add vlan id in get_subnet_* methods

Standardize vlan id fetching in all methods

* ipam-client: add get_subnet_with_ips method

This allows to fetch a whole subnet with all allocated
ip addresses.

* use a buffered cursor for mysql

This prevents "unread result" issues

* handle /0 subnets

DB will return an empty string, translate it to 0