Make body of std.email optional
Make body of std.email optional

According to SMTP protocol both the subject and body of the email
can be blank. Hence, making the body of std.email as optional.

Change-Id: I1a2551467247a46c4e3fa5425a367d0f83756b9c
Closes-Bug: #1649366

openstack/mistral,StackStorm/mistral,openstack/mistral,StackStorm/mistral