Fix OpenApi ignoring min-elements for list

Fix OpenApi ignoring min-elements for list

If list have min-elements parameter, it is ignored and example with
only one element is created.
Edited definitionGenerator to adhere to the amount of min-elements,
also if parameter is defined as key, unique examples are created for
string and number types.

MinItems and maxItems values are missing in list property,
so they are added.

JIRA: NETCONF-1172
Change-Id: I5411fe6dd451a87fd7bfa53eb47206685aa624e7
Signed-off-by: Matej Sramcik <matej.sramcik@pantheon.tech>
Signed-off-by: Peter Suna <peter.suna@pantheon.tech>