JIRA: https://jira.spring.io/browse/INT-2543
We may have a mail server host configured in the properties and the
target session will resolve it properly from provided `javaMailProperties`.
The same applies for `username`
* Do not require `host` and `username`, when `javaMailProperties` is
provided
* Add additional factory methods into the `Mail` factory for Java DSL
* Add Java DSL sample into the `mail.adoc`
a5e437f94a