Plugins: Add isEnabled method

Plugins are now disabled at startup time unless there is some relevant
configuration in hydra.conf. This avoids hydra-notify having to do a
lot of redundant work (a lot of plugins did a lot of database queries
*before* deciding they were disabled).

Note: BitBucketStatus users will need to add 'enable_bitbucket_status
= 1' to hydra.conf.

src/lib/Hydra/Plugin/CoverityScan.pm

@@ -6,6 +6,11 @@ use File::Basename;
 use LWP::UserAgent;
 use Hydra::Helper::CatalystUtils;
 
+sub isEnabled {
+    my ($self) = @_;
+    return defined $self->{config}->{coverityscan};
+}
+
 sub buildFinished {
     my ($self, $b, $dependents) = @_;