Remove size default for thinpool

The needed size depends on many factors that mock cannot predict.
I think it's better to require the user to always specify the size.
Implement sharing LVM thinpool between independent configs

There's a new 'pool_name' config option that allows to specify the thinpool name
which will be used. When multiple configs specify the same pool_name, the same
thinpool will be used. Namespacing snapshots/heads should ensure peaceful
coexistence.