The 'tag' pieform element will now become a wrapper for the
autocomplete one - rather than using the 'autocomplete' element
directly - so that we can centeralise how the tags field is generated and
avoid having to duplicate setup code.

Todo: need to get the select2 behat test to do ajax search

Change-Id: I795969f2411ed5a088103398460e6ac135015319
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>