functional: introduce a new test TestReconfigureServer for fleetd
functional: introduce a new test TestReconfigureServer for fleetd

A new test TestReconfigureServer() checks whether fleetd managed to
keep its net.Listeners across reconfiguration of fleetd, after receiving
SIGHUP.

giantswarm/fleet,giantswarm/fleet,coreos/fleet,jonboulle/fleet,giantswarm/fleet,dongsupark/fleet,coreos/fleet,endocode/fleet,jonboulle/fleet,dongsupark/fleet,endocode/fleet,endocode/fleet,jonboulle/fleet,coreos/fleet,dalbani/fleet,dalbani/fleet,dongsupark/fleet,dalbani/fleet