Set block indentation to 1 on iosxr templates (#21105)

Network devices in the lab have by default an indentation of 1 for
sub-sections, and so does the netcfg.NetworkConfig constructor
indent param.
This will fix reported issue 21055, and make the integration tests
to pass.

Fixes #21055

test/integration/targets/iosxr_template/templates/basic/config.j2

@@ -1,5 +1,5 @@
 hostname {{ inventory_hostname_short }}
 !
 interface Loopback999
-   description this is a test
-   shutdown
+ description this is a test
+ shutdown