add support of cluster "rules"

[benmaier]

A user should be able to add a cluster rule and potentially make it default. this will make it easier to initialize qsuite directories for different clusters. e.g.

$ qsuite init SGE

should give you your default config file for the configuration of your SGE cluster, while

$ qsuite init PBS

should do the same for your PBS cluster. User should also be able to add own rules. for example

$ qsuite init physik