Fix logic of POST schema reference creation

Fix logic of POST schema reference creation

After implementing the 'Use first child in POST payloads' changes,
list child nodes no longer include a 'key' value in example payloads.

Removed unnecessary 'post' prefix from schema references
for lists and containers child nodes in POST requests,
enabling the use of a valid schema.

JIRA: NETCONF-1054
Change-Id: I26a07e1ad04a0d08567c31d48d24f19349dd8022
Signed-off-by: Yaroslav Lastivka <yaroslav.lastivka@pantheon.tech>
(cherry picked from commit 65c1945627ad388d4239f4a951aa9d4efc2dba74)