Add @Conditionals to permit JPA/Mongo mixed usage

I decided to go with both approaches (make the autoconfig for
repositories @ConditionalOnMissingBean(RepositoryFactoryBeanSupport),
so the first one wins; and also make them conditional on
spring.data.*.repositories.enabled=true. The ordering problem
is still there really (it's not defined which repositories will
be created by the autoconfig), so if a user is going to have
2 repository implementations on the classpath, he is going to
have to either choose one to disable, or manualy @Enable* the
other one.

Fixes gh-1042