Add optional key argument to display subtree of pillar data

The output of pillar.data comprises a substantial chunk of data.
By introducing key selection on the dictionary, we can display
the subtree of immediate interest.

Inadequacy: Should allow for arbitrary keying into subsections
of the data. Current change is the simplest useful improvement.
Merge pull request #3707 from sbworth/develop

Add optional key argument to display subtree of pillar data