Cleaned up filesystem loading code
Don't touch non-automatic btrfs snapshots

Added sanity check to btrfs.get_snapshots() to disregard
non-automatic snapshots
Made CONFIG_FILE global variable in btrfs module
Added is_enabled() to filesystem modules.

Filesystem modules must be able to determine if they are enabled at load
time.  Filesystems that return False for is_enabled() will not be loaded
into tardis.filesystems
Only load enabled filesystems

tardis.filesystems will only load fs modules into itself if they are
return True for <module>.is_enabled()
Set default is_enabled() value to False

Filesystem modules that do not implement is_enabled() are prevented from
loading