[Fiware-tech-help] [Chp - IoT][IDAS] Add mongodb configuration information

[David Muriel]

There is no information on the installation guide of the required
configuration for the mongodb server.  From the examples of the
variables found on the neoidas-monitbd and neoidas-bd sections it can
be deduced that the platform may need a specific mongodb configuration
prior to install (from the examples, a replica set configuration).
This is not explained on the guide and should be.

If the platform can be installed with just a single mongodb node, then
add the necessary information to the guide for the user to be able to
install and configure it.  Also, if a mongodb replica set is a
requirement for the platform, provide instructions on how to install
this configuration.