tools: Fixing mailserver validation

This solves mail server validation -
In case MAIL_SERVER does not exist, a proper
error will be reported indicating MAIL_SERVER should
exist.
In case MAIL_SERVER exists, but empty or
with a non resolvable content, a proper error
indicating the MAIL_SERVER should contian resolvable
address will be reported.

Change-Id: Id78bec65bfa883e5fc7ec369a8b4beba6610b4d0
Bug-Url: https://bugzilla.redhat.com/921405
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>