domainMonitor: Rename confusing lastCheck variable

In repoStats we use the "lastCheck" value as the time since domain
status was checked. In DomainMonitorStatus we use "lastCheck" value as
the time were domain was checked.  This patch eliminates the confusion
by using "checkTime" for the time where status was checked.

Change-Id: I56ce7c2903c33c25693601dde5a1af376b789e59
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21879
Reviewed-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>