Create Unit Test for POST existing data error

Create Unit Test for POST existing data error

Successfully create resource of your choice using POST request.
When we invoke the same request again, we get the error
response 500 Internal Server Error. According to RFC 8040
we should get 409 Conflict.

Unit Test has been created to verify this issue.

JIRA: NETCONF-1130
Change-Id: Iad1254d2f058a8aa69ed1076e43823224d54c06e
Signed-off-by: lubos-cicut <lubos.cicut@pantheon.tech>
Signed-off-by: Yaroslav Lastivka <yaroslav.lastivka@pantheon.tech>
Signed-off-by: Ivan Hrasko <ivan.hrasko@pantheon.tech>