With the change of requiring to configure account settings properly by
using affix settings, we forgot another special snowflake, namely the
configuration of mail properties, which can be arbitrary in the
configuration. Those properties are used when an email is sent.
This commit adds a few (but not all of those) options back and removes
the link in the documentation to refer to all of those settings.
Some settings are useless, as they only change the execution
expectations when a mail is sent, which the watch has control over.
The following settings are supported
* smtp.{host,port,user,password}
* smtp.auth
* smtp.starttls.{enable,required}
* smtp.{timeout,connection_timeout,write_timeout}
* smtp.{local_address,local_port}
* smtp.send_partial
* smtp.wait_on_quit
relates elastic/x-pack-elasticsearch#4048
Original commit: elastic/x-pack-elasticsearch@39d5624710
6bd5e9ef91