add startsector for partitions

README.rst

                   mount: ${linux:storage:mount:data}
 
 Create partitions on disk. Specify size in MB. It expects empty
-disk without any existing partitions.
+disk without any existing partitions. (set startsector=1, if you want to start partitions from 2048)
 
 .. code-block:: yaml
 
         storage:
           disk:
             first_drive:
+              startsector: 1
               name: /dev/loop1
               type: gpt
               partitions:

linux/storage/disk.sls

     - pkg: parted
 
 {% set end_size = 0 -%}
+{% if disk.get('startsector', None) %}
+{% set end_size = disk.get('startsector')|int %}
+{% endif %}
 
 {%- for partition in disk.get('partitions', []) %}