stack/slapos: Version up

slapos.core to 1.5.6 and slapos.toolbox to 0.104:

 * cleanup stale monitor files and stale promises
 * support download-from-binary-cache-force-url-list option

/reviewed-on !679

stack/slapos.cfg

@@ -138,14 +138,14 @@ pytz = 2016.10
 requests = 2.13.0
 six = 1.12.0
 slapos.cookbook = 1.0.123
-slapos.core = 1.5.5
+slapos.core = 1.5.6
 slapos.extension.strip = 0.4
 slapos.extension.shared = 1.0
 slapos.libnetworkcache = 0.20
 slapos.rebootstrap = 4.2
 slapos.recipe.build = 0.42
 slapos.recipe.cmmi = 0.12
-slapos.toolbox = 0.103
+slapos.toolbox = 0.104
 stevedore = 1.21.0
 subprocess32 = 3.5.3
 unicodecsv = 0.14.1