cvmfs_swissknife info check for GC'ability
store stratum1 url in server.conf
FIX: cvmfs_server snapshot sanity check
safe guard open_transaction() on stratum1
configure GC'ability in cvmfs_server add-replica
cvmfs_server gc transaction disableable
allow GC in stratum 1 replicas
TEST/FIX: load_repo_config() for stratum 1
TEST: garbage collection in a stratum 1 repository
QUICK-FIX: goto cleanup
FIX: cvmfs_server gc -k on stratum1
Merge pull request #618 from reneme/feature-gc_on_s1

Feature: Garbage Collection on Stratum 1 Replica

This allows to use the garbage collection on a stratum 1 replica.

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...)