Allow run all full backups

Allow set `full_backup_depth` to 0, so able to do pure full backups
every time.
diff --git a/staffeln/conductor/backup.py b/staffeln/conductor/backup.py
index 9f3f54d..4be99f1 100755
--- a/staffeln/conductor/backup.py
+++ b/staffeln/conductor/backup.py
@@ -286,6 +286,8 @@
         """
         # select * from backup order by Id DESC LIMIT 2;
         try:
+            if CONF.conductor.full_backup_depth == 0:
+                return False
             backups = self.get_backups(
                 filters={"volume_id__eq": volume_id},
                 limit=CONF.conductor.full_backup_depth,
diff --git a/staffeln/conf/conductor.py b/staffeln/conf/conductor.py
index 1032ab4..b07b956 100755
--- a/staffeln/conf/conductor.py
+++ b/staffeln/conf/conductor.py
@@ -27,7 +27,7 @@
         "report_period",
         default=1440,
         min=10,
-        help=_("The time of report period, the unit is one second."),
+        help=_("The time of report period, the unit is one minute."),
     ),
     cfg.StrOpt(
         "backup_cycle_timout",
@@ -58,7 +58,7 @@
     cfg.IntOpt(
         "full_backup_depth",
         default=2,
-        min=1,
+        min=0,
         help=_("Number of incremental backups between full backups."),
     ),
 ]