Missing validate_config_options function error (Bug #1336111)

Problem is the line 1809 in lib/mahara.php namely
- [optional] validate_config_options() was not being respected

So I added a check to see if the function exists for the plugin before
trying to call it.

Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>

Change-Id: Iceabeec423f67339903959fe4083bb87250cad12
Merge "Missing validate_config_options function error (Bug #1336111)"