Allow to specify custom horizon.tables.DataTable template

This fix allows user to provide his own template in Meta inner
class of DataTable's subclass (and thus extend or completely
override the default 'horizon/common/_data_table.html' template).

Change-Id: I1d1a7b2bf74dcaaee7a7096521853920feb40b21
Closes-bug: #1233609
Merge "Allow to specify custom horizon.tables.DataTable template"