functional: introduce a test TestFleetctlWithEnv
functional: introduce a test TestFleetctlWithEnv

TestFleetctlWithEnv runs simply fleetctl list-machines, but by setting
an environment variable FLEETCTL_ENDPOINT, instead of the cmdline option
'--endpoint'. This way functional test is able to detect regressions
like https://github.com/coreos/fleet/issues/1631.

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