lib: Correct bug for TE metric wrong assignement

lib: Correct bug for TE metric wrong assignement

When link-param is enabled for a given interface, TE metric is automatically
assigned to the metric of the interface. However, the metric of the interface
could be unassigned and keep the default value equal to 0. Thus, if the TE
metric is not explicitely modified within the `link-param metric` statement,
TE metric remains set to 0 which is not a valid value especially when
computing constrainted path.

This patch changes the assignement of the default value of the TE metric.
It is set to the metric of the interface only if the latter is not equal to 0.
TE topotests for OSPF and IS-IS have been adjusted accordingly.

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>