Ktest: Add email support

Ktest: Add email support

Users can define optional variables to get email notifications.
Ktest can send emails when the script:
 * was started
 * failed with fatal errors and called dodie()
 * completed all testing

Users have to setup the mailer provided in config prior to using this script.
Supported mailers: mailx, mail, sendmail
mailer specific routines are _sendmail_send(), _mailx_send()

Link: http://lkml.kernel.org/r/1522094884-22718-2-git-send-email-tianyang.chen@oracle.com
Suggested-by: Dhaval Giani <dhaval.giani@oracle.com>
Signed-off-by: Tim Tianyang Chen <tianyang.chen@oracle.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>