Do not issue duplicate warnings for lists missing keys

Do not issue duplicate warnings for lists missing keys

We currently are issuing the config list warning for each instantiated
site, which ends up flooding our logs, as BGPCEP takes advantage of our
lenience -- leading to 3400+ warnings.

Make sure we flag each original list only once, suppressing other
instances of the violation. This presumably will provide enough guidance
while keeping the noise to a sane amount (~200 warnings).

JIRA: YANGTOOLS-1090
Change-Id: I2488ad7144c1827f7689d496669e4c97b62b0570
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 1fb451b2a874cf198195895be035478cd9aa1635)