Fix consistency between Order/Secret/Container

Order/Secret capitalized factory methods disappear and are replaced
by more appropriate get/create methods in their respective managers,
to match the revised Container code.

The higher level REST functions for get/get_raw/post/delete become
private so that the namespace for developers using the client is
much cleaner. For example, nothing is visible in the top level
client object besides the manager classes for container/secret/order
and nothing is visible inside the manager classes besides the
appropriate get/create/delete methods (or other applicable
functions).

Internally, the barbican_cli code uses at least one private property
from ContainerManager, but it is assumed to be OK since it is within
the same "package", and the code will be maintained simultaneously.

Change-Id: I89525506f3be26b77421a5b8efa49bb645169aaf