Content type support in Email mapper

[potiuk]

Is your feature request related to a problem? Please describe.
Unfortunately, currently the EmailOperator only accepts the mime_subtype parameter.
However it only works for multipart subtypes, as the operator appends the subtype
to the multipart/ prefix. Therefore passing either html or plain from Oozie makes no sense.

As a result the email will always be sent with the EmailOperator's default Content-Type value,
which is multipart/mixed.

Describe the solution you'd like
Extends the EmailOperator to support content type as well.