Changed thumbnail enabling key to control pdf only.

Conflicts:
	src/main/java/org/linagora/linshare/core/business/service/impl/DocumentEntryBusinessServiceImpl.java
	src/main/resources/springContext-business-service.xml
Add keys to disable thumbnails for pdf and non-pdf

Conflicts:
	src/main/java/org/linagora/linshare/core/business/service/impl/DocumentEntryBusinessServiceImpl.java
	src/main/resources/springContext-business-service.xml
Fix thumbnail disabling