Fix Illumina flowcells query to use the entity_type column

[kjsanger]

The query to find Illumina flowcells in the MLWH by component was
using tag index to try to identify controls. This is not a reliable
method; entity_type should be used instead. The set of valid entity
types is defined in the metadata of the MLWH schema.

Builds on #231 , which should be merged first.

[mgcam]

The position attribute of the component object is not optional, therefore the condition is not required. Also see the type definition for the position (lane) in Perl code

The iseq_product_metrics table does contain rows with NULL position value. However, these rows do not correspond to the components