Add skip support to the plugin

It is a common practice for maven plugins to add a "skip" property that allows for control of the execution of the plugin based on the value of a property.  Without it the only way to limit execution of the plugin is the use of profiles which is not always the best solution to a problem.

This PR adds the skip property (cf.skip) to the AbstractCloudFoundryMojo and should not error out even if no configuration is specified.