get_subnet method (#38)

* Add add_subnet in the AbstractIPAM class

* Since it's implemented in the PHPIPAM class and already tested
* It should probably be present in the abstract class

Change-Id: Ia9dc1f9364cd6f3f955865238e5b8092ef48345a

* Add get_subnet method

* There was not method to retrieve a subnet from the ip_network
  information (network_address + prefixlen)
* Only from a description or with the list of IPs. The later method
  returns nothing if the subnet exists with no IP allocated. We can't
  guess if the subnet is empty or absent

Change-Id: I6c05d572135da326088a3c9634bdcd56cc505405

* Pin mysql-connector-python to keep Python 2.7 compat

* mysql-connector-python 8.0.24 breaks the compatibility with Python 2.7
* This is unfortunate but we still need to keep it

Change-Id: I01945503264299819cf569b5377e5a364483d4ea

Co-authored-by: Arnaud Chong <[email protected]>