This commit adds support for configuring the JSP servlet’s init
parameters via the environment using server.jsp-servlet.init-parameters.*.
As part of this change the configuration of registerJspServlet and
jspServletClassName have been moved onto a new type, JspServlet, and the
existing setters on ConfigurableEmbeddedServletContainer have been 
deprecated. In addition to providing a model for configuring the JSP
servlet that’s consistent with the model for other configuration (SSL,
for example), this change also means that the class name and whether or
not the servlet is registered at all can now also be configured via the
environment.

Closes gh-2825