Fix per-check interval/cached metrics

Credit: Michal Racek <michal.racek@t-systems.sk>

In Resmon 1, it was possible to add an interval parameter to an individual
check to give it a larger check interval than regular checks (cached results
would be returned in the meantime). This functionality wasn't properly carried
over into resmon 2, and this commit fixes that.

lib/Resmon/Module.pm

@@ -28,9 +28,9 @@ sub cache_metrics {
 
 sub get_cached_metrics {
     my $self = shift;
-    return undef unless $self->{check_interval};
+    return undef unless $self->{config}->{interval};
     my $now = time;
-    if(($self->{lastupdate} + $self->{check_interval}) >= $now) {
+    if(($self->{lastupdate} + $self->{config}->{interval}) >= $now) {
         return $self->{lastmetrics};
     }
     return undef;