Provide a get_secret library method

Currently we do not provide a mechanism for a user to specify an
alternate 'secrets' data bag. By adding a get_secret method, we can
support this feature request without breaking other dependent
cookbooks. Once this method is accepted, we can retrofit all calls
to 'secret' with 'get_secret'.

New tests added and all tests pass.

Changelog and metadata updated to semver 8.3.0.

Change-Id: Id2adaf7bc197b3d7fd94c5c9614fda145a8501f0
Closes-Bug: #1288784