Make ES Heap Size configurable.

wiki.o.o is much smaller than our typical elasticsearch nodes. For it to
use elasticsearch it needs to allocate a much smaller heap. Make the
heap size configurable (keep the 16g default) and set the heap size to
1g for wiki.o.o.

Change-Id: I0c5fb4865eb07c0fa5bbe4cf698a9f0e9ea9f2fc