calico: fix NetworkManager check (#7169)

Previous check for presence of NM assumed "systemctl show
NetworkManager" would exit with a nonzero status code, which seems not
the case anymore with recent Flatcar Container Linux.

This new check also checks the activeness of network manager, as
`is-active` implies presence.

Signed-off-by Jorik Jonker <jorik@kippendief.biz>