Configuration option to tweak SubnetLen

[Impact]

Environments with big machines to run kubernetes-worker or a high number of create/destroy pods will see a starvation of available IP address since a /24 subnet (flannel's default) will be able to have 254 hosts, flannel uses a 24h lease which will make this problem easier to find.

Currently the configuration passed omits the SubnetLen key:

https://github.com/charmed-kubernetes/charm-flannel/blob/master/reactive/flannel.py#L178

This should be exposed in a configuration option, and in my opinion the charm should set this to 22 by default since it would allow environments running CI/CD jobs (e.g. gitlab's ci runner) to run without problems.

Using a /22 per worker and a /16 for the cluster would give space to have 64 workers and 1022 IP addresses per worker.

 ~ $ ipcalc-ng 10.1.0.0/16 --split 22 |grep Network | wc -l
64
 ~ $ ipcalc-ng 10.1.252.0/22
Network: 10.1.252.0/22
Netmask: 255.255.252.0 = 22
Broadcast: 10.1.255.255

Address space: Private Use
HostMin: 10.1.252.1
HostMax: 10.1.255.254
Hosts/Net: 1022

[Workaround]

This script could be used to set the SubnetLen manully - https://paste.ubuntu.com/p/Bzr83SCb9H/ - be aware that the charm may overwrite the change, so you may need to install a cronjob to have a consistent configuration over time.