tools: Adds more validations to MAIL_PORT in notifier

Adds more validations to MAIL_PORT option in notifier's configuration.
MAIL_PORT has to be an integer from 1 to 65535 inclusively.

Change-Id: I770d1636dc25c0c9b0076a92dcac20ddec551acf
Bug-Url: https://bugzilla.redhat.com/1051061
Bug-Url: https://bugzilla.redhat.com/1051048
Signed-off-by: Martin Perina <mperina@redhat.com>