Add "rcptid" to the list of PKs when moving users between clusters

Was erroring with "can't find index for table vgift_trans"; fix was just
to make sure that we acknowledged that "rcptid" in vgift_trans is also a
userid-type primary key (which identifies the user to be moved between
clusters)

Also refactors so that the list of userid-type primary key columns is a
variable, to avoid having to add to two if statements
Merge pull request #779 from afuna/movucluster-failing-vgift-trans

Add "rcptid" to the list of PKs when moving users between clusters