Content-Type can now be set for email-ext publisher

The email-ext publisher was always sending email using the default
content type which is server wide.  This tiny patch let us overwrite the
content-type on a job basis.

The YAML possible values are 'html' or 'text' to force the content-type
to one of the two modes.

Change-Id: Id66bc3369332c389dfb6318113392694a4f98aad
Merge "Content-Type can now be set for email-ext publisher"