Fix eos entity lookups with YangInstanceIdentifier

Caused by entity-name only being a string type which was inadvertently
trying to lookup all entities as General.

Fix this by changing entity-name to a union of instance-identifier and
string.

Also changed up the output of get-entities slightly:
- if the entity is string based it will only contain the name in the
  output.
- if, however, it is Instance-identifier based it will contain the
  entire identifier in the name field.

JIRA: CONTROLLER-2009
Change-Id: I93e108bd54b14850b6bf276ec1eca4d7671d7d87
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>