sdk: Add method to get entity by string id

Currently the method that allows retrieving an entity by id assumes that
the identifier is a UUID, but this isn't true for some entities. This
patch adds an additional method that allows retrieving entities by id
using a string as the parameter.

Change-Id: I8fb3fd8e6b59f0110a091240bc089e70d3c39498
Bug-Url: https://bugzilla.redhat.com/1099818
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>