Feature: Garbage Collection on Stratum 1 Replica

This allows to use the garbage collection on a stratum 1 replica. Unfortunately it was less of a no-brainer than I thought.

Until now a stratum1 didn't know its local HTTP URL, because it didn't need to know. Hence, now one can specify this (optionally) in `cvmfs_server add-replica -w`, otherwise the default `http://localhost/cvmfs/${replica_alias}` is used analogous to the stratum0 generation. Furthermore the stratum1 can now be configured as being *garbage collectable* using `CVMFS_GARBAGE_COLLECTION=true`. This configuration is automatically picked up if the replicated stratum0 is garbage collectable. That said, `cvmfs_swissknife info -g` determines if a repository (both stratum0 and stratum1) is configured as being garbage collectable. 

Also this (quick-)fixes a compiler error due to `goto` jumping over variable declarations in `cvmfs_swissknife pull` and contains an integration test for the basic stratum1 garbage collection. (More tests to come later...)