Bug 27424: Add ability to specify an SMTP server in the database as the default server

Bug 27424: Add ability to specify an SMTP server in the database as the default server

Bug 22343 adds the ability to defined SMTP servers via the UI; But to then utilise them you have to go to each individual library via the libraries admin area and select the SMTP server.

We should have a way to override the fallback/default SMTP server right from the SMTP servers administration page.. setting one of our defined SMTP Servers as the system default rather than using the hard coded fallback options.

Test Plan:
1) Apply this patch set
2) Restart all the things!
3) Browser to the SMTP servers editor,
   verify only one server can be set as the default server
4) Set a default server, verify that server was used to send email from
   a cronjob, AND/OR prove t/db_dependent/Koha/SMTP/Server.t

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>