title: FlywayAutoConfiguration doesn't "see" data source
body: <p>Using spring boot 2.0.0.M5 with web starter and flywaydb on the classpath.
  The auto-configuration debug output on one hand shows there's a data source configured as expected:</p>
  
  <pre><code>DataSourceAutoConfiguration matched:
    - @ConditionalOnClass found required classes 'javax.sql.DataSource', 'org.springframework.jdbc.datasource.embedded.EmbeddedDatabaseType'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
  
  DataSourceAutoConfiguration.PooledDataSourceConfiguration matched:
    - AnyNestedCondition 1 matched 1 did not; NestedCondition on DataSourceAutoConfiguration.PooledDataSourceCondition.PooledDataSourceAvailable PooledDataSource found supported DataSource; NestedCondition on DataSourceAutoConfiguration.PooledDataSourceCondition.ExplicitType @ConditionalOnProperty (spring.datasource.type) did not find property 'type' (DataSourceAutoConfiguration.PooledDataSourceCondition)
    - @ConditionalOnMissingBean (types: javax.sql.DataSource,javax.sql.XADataSource; SearchStrategy: all) did not find any beans (OnBeanCondition)
  
  DataSourceConfiguration.Hikari matched:
    - @ConditionalOnClass found required class 'com.zaxxer.hikari.HikariDataSource'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
    - @ConditionalOnProperty (spring.datasource.type=com.zaxxer.hikari.HikariDataSource) matched (OnPropertyCondition)
  </code></pre>
  
  <p>On the other hand, the auto-config fails to configure <code>FlywayAutoConfiguration</code> due to "missing (?!)" data source:</p>
  
  <pre><code>FlywayAutoConfiguration:
    Did not match:
       - @ConditionalOnBean (types: javax.sql.DataSource; SearchStrategy: all) did not find any beans of type javax.sql.DataSource (OnBeanCondition)
    Matched:
       - @ConditionalOnClass found required class 'org.flywaydb.core.Flyway'; @ConditionalOnMissingClass did not find unwanted class (OnClassCondition)
       - @ConditionalOnProperty (spring.flyway.enabled) matched (OnPropertyCondition)
  </code></pre>
  
  <p>Any suggestions how to fix (or at least further debug) this will be greatly appreciated!</p>