We don't merge common_attrs with node's custom attributes
in deployment info. But YAQL expressions still need to
access 'unified' deployment info, related to node. So
we were merging dicts every time yaql want to access it.
It's a bit slow, so this patch introduces class, which
works like union of several dicts without creation of the new
data structure.
Partial-Bug: #1596987
Change-Id: Iea32ab222421fc7a3c5df66e7e48f4d1a4b931f5