Add YDK Meta Data for must and when statements in a contaner

[xulleon]

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Expected Behavior

Currently YDK meta data does not have a field to indicate if a container has a "must"
statement.

          container segment-routing {
            presence "Indicates a segment-routing node is configured.";
            description "Segment Routing configuration";
            must "not(mpls and disable)";
            container mpls {
            …

            container forwarding {
              presence "Indicates a forwarding node is configured.";
              csc:xr-sch-pathname "/cfg/gl/ipv4-ospf/proc/<224>/ord_f/default/ord_v/area/<101>,<102>/ord_d/area_scope/ord_f/sr_forw";
              csc:xr-task "ospf";
              description "Enable Segment-routing forwarding on interfaces";
              must "mpls or disable";
              container mpls {

Current Behavior

Steps to Reproduce

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System Information

[ygorelik]

Implemented in releases 0.5.5.post2 and 0.8.4