server: switch to h2 as default

Make Kill Bill use h2 instead of MySQL as a default database.
This makes it easier to get started with Kill Bill.

Note that multi-nodes deployments are not supported yet on H2.

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
.idea: remove duplicate modules

Signed-off-by: Pierre-Alexandre Meyer <pierre@mouraf.org>
Merge branch 'master' of github.com:killbill/killbill