functional: call systemctl is-enabled instead of list-unit-files
functional: call systemctl is-enabled instead of list-unit-files

TestSingleNodeConnectivityLoss has been so far retrieving is-enabled
status by calling "systemctl list-unit-files UNITS". It was expected
to get "enabled" for each unit. This has worked until systemd v227.
However, that's not the case any more since systemd v228, precisely
since https://github.com/systemd/systemd/commit/0ec0deaa30d0e68430f03fa6f32affa576481d18
("install: follow unit file symlinks in /usr, but not /etc when looking
for [Install] data"). Since then, systemctl list-unit-files returns
"linked-runtime" status for each fleet unit, which is actually
reasonable behavior for units that are not installed persistently
under /etc/systemd.

With systemd v228 or newer, however, TestSingleNodeConnectivityLoss
in fleet functional tests cannot make use of "systemctl list-unit-files"
any more. And it would not be optimal to simply change enabled to
linked-runtime, because then the test couldn't work with systemd < v228.

So let's work around it, by calling "systemctl is-enabled UNIT" instead.
That command has returned correct status "linked-runtime", no matter
whether systemd >= 228 or not.

Fixes: https://github.com/coreos/fleet/issues/1588

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