Allow to configure CIDR exclusion blocks

It could be useful to be able to express exclusion CIDR blocks when configuring firewalls. For example, CloudSigma allows to configure firewall rules with something like `!192.168.0.0/24` to allow traffic from any IP address outside that range.

This change should also simplify a lot that provider and unlock https://github.com/jclouds/jclouds-labs/pull/70
I'm not sure why this is configured this way, but the skip method reads [this variable](https://github.com/jclouds/jclouds/blob/master/compute/src/test/java/org/jclouds/compute/extensions/internal/BaseSecurityGroupExtensionLiveTest.java#L67), which is not set by checking the presence of the extension. Seems like something that can be rethought and refactored in an upcoming PR.