standalone: fix support of recursive slapos

When having more than one read_only_shared_part_list, the template was
first rendered with indentation and then dedented, but since dedent only
remove the common indentation, we ended up with slapos.cfg like this:

```
$ cat /data/slappart11_testnode/cqg/inst/test0-9/tmp/etc/slapos.cfg

            [slapos]
            software_root = /data/slappart11_testnode/cqg/inst/test0-9/tmp/soft
            instance_root = /data/slappart11_testnode/cqg/inst/test0-9/tmp/inst
            shared_part_list =
              /srv/slapgrid/slappart11/srv/shared
/srv/slapgrid/slappart11/srv/testnode/cqg/shared
              /data/slappart11_testnode/cqg/inst/test0-9/tmp/shared
            master_url = http://10.0.12.198:46312
            computer_id = local
            root_check = False
```

First dedent, then render the template to prevent this issue. Apply this to
other uses of textwrap.dedent for consistency.

/reviewed-on !140