return the underlying string value without schema type based conversion

[kwangsuk]

RCA - in case a value is encoded as string type, e.g. keepalive-interval (decimal64), the underlying value needs returned without schema type based conversion. otherwise, it cause crash due to the type assertion. Fix - return the stringified value if the underlying type of interface value is string

sonic# configure terminal
sonic(config)# router bgp 65100
sonic(config-router-bgp)# timers 2 3