Update security_considerations doc

Update security_considerations doc

 - Disabling the remote shutdown port throws an exception if
   it is set in `etc/config.properties`.
 - HTTPS configuration should be set in `etc/org.ops4j.pax.web.cfg`
 - Removing instructions to edit the Jetty.xml file. These instructions
   are misleading. The provided configuration only sets up the HTTP
   connector. HTTPS works fine with the default jetty.xml configuration
   provided by odlparent.
 - Correcting the verification request to use RFC8040 and "operations"
   request instead of "modules", which do not work without a module
   identifier.

JIRA: DOCS-152
Change-Id: Ia32166e3ec627bd49d1af9cbf1385b85bf52b23a
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>