janus-gateway: make echo test plugin optional

janus-gateway: make echo test plugin optional

Currently janus-gateway is built by default with the echo test plugin.
This causes a example config file to be placed on the target filing system
that the user may not need or want, creates a 36K .so
file on the target directory, and janus will also generate a warning when
ran if the plugin does not have a matching configuration file setup by
the user, as it will not use the example config file by default.

This patch makes the echo test plugin optional.

Signed-off-by: Adam Duskett <aduskett@codeblue.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>