add (optional) manifest field 'G'
create garbage collectable repo
FIX: sustain GC flag after publish
check for enabled garbage collection
check GC setting in swissknife
FIX: test_functions create_repo() extra options
TEST: adapt GC integration tests
add toggle_gc() to test_functions
TEST: check disabling of GC
Merge pull request #616 from reneme/feature-gc_safeguard

Feature: Garbage Collection can be Disabled

This adds a configuration flag (`CVMFS_GARBAGE_COLLECTION`) to enable the garbage collection feature. By default, it is disabled and needs to be enabled explicitly. Furthermore this adds a field to the `.cvmfspublished` manifest file to mark a repository as garbage collectable.

The pull request also adapts the garbage collection integration tests and adds one additional integration test to verify that `CVMFS_GARBAGE_COLLECTION` properly disables the feature.