Distributed MooseMesh::getBoundaryIDs(name)
Distributed MooseMesh::getBoundaryIDs(name)

We may not have all our boundary IDs locally, but userspace code is
probably expecting (in some cases definitely expecting) to be querying
the global boundary ID set, so let's use a global union to get it.

This fixes a dgkernels/dg_block_restrict.1D_test #1500 regression for
me with --distributed-mesh on 2 processors.

harterj/moose,bwspenc/moose,liuwenf/moose,liuwenf/moose,friedmud/moose,dschwen/moose,lindsayad/moose,Chuban/moose,sapitts/moose,Chuban/moose,backmari/moose,bwspenc/moose,liuwenf/moose,jessecarterMOOSE/moose,permcody/moose,SudiptaBiswas/moose,laagesen/moose,idaholab/moose,lindsayad/moose,yipenggao/moose,permcody/moose,nuclear-wizard/moose,lindsayad/moose,YaqiWang/moose,YaqiWang/moose,SudiptaBiswas/moose,bwspenc/moose,idaholab/moose,sapitts/moose,jessecarterMOOSE/moose,laagesen/moose,friedmud/moose,milljm/moose,milljm/moose,idaholab/moose,laagesen/moose,idaholab/moose,yipenggao/moose,liuwenf/moose,milljm/moose,liuwenf/moose,dschwen/moose,laagesen/moose,SudiptaBiswas/moose,backmari/moose,laagesen/moose,nuclear-wizard/moose,permcody/moose,sapitts/moose,nuclear-wizard/moose,friedmud/moose,andrsd/moose,jessecarterMOOSE/moose,dschwen/moose,liuwenf/moose,milljm/moose,harterj/moose,bwspenc/moose,jessecarterMOOSE/moose,nuclear-wizard/moose,harterj/moose,harterj/moose,friedmud/moose,SudiptaBiswas/moose,sapitts/moose,SudiptaBiswas/moose,sapitts/moose,yipenggao/moose,dschwen/moose,backmari/moose,dschwen/moose,Chuban/moose,Chuban/moose,permcody/moose,andrsd/moose,andrsd/moose,milljm/moose,andrsd/moose,YaqiWang/moose,lindsayad/moose,lindsayad/moose,andrsd/moose,bwspenc/moose,harterj/moose,backmari/moose,jessecarterMOOSE/moose,idaholab/moose,yipenggao/moose,YaqiWang/moose