networkd: make metric of routes configurable

networkd: make metric of routes configurable

Now route metric can be configuted via conf file:

example conf:

[Match]
Name=em1

[Route]
Gateway=192.168.1.12
Metric=10

Test:
ip route output
default via 192.168.1.12 dev em1 metric 10

[tomegun: squash TODO update and reword man page a bit]