Added liquibase autoconfiguration for database migrations

If Liquibase is on the classpath it will fire up on startup. Various
config options are available (as well as the option to disable it).
Liquibase uses a YAML format for changes (in classpath:db/changelog).
Add autoconfig support for Flyway migrations

Flyway starts up with its default settings if it is on the classpath.
You can also ask Boot to barf if the migration scripts are missing.

Fixes gh-370
Drop 'spring' from 'spring.[liquibase,flyway]'
Harmonize enabled flags for flyway and liquibase
Downgrade Liquibase to 3.0.8

There are some issues with 3.1.1. Performance is one
(according to JIRA https://liquibase.jira.com/browse/CORE-1706),
but the biggest is that there is a package change for the main
ChangeLogHistoryService provider and it's package
(liquibase.channgelog) is not included in the service locator
scan by default. For some reason this only manifests itself
when using the SpringPackageScanClassResolver.

Fixes gh-687