Use linux.system.shell state

https://gerrit.mcp.mirantis.com/25351/ was merged but linux.system.shell
state wasn't included into init.yml and was never used.

This commit fixes this.

Related-Prod: PROD-23581

Change-Id: I89e09247dd2566b8a5b0c0e67e8ca9c789ed57f6

linux/system/init.sls

 include:
 - linux.system.env
 - linux.system.profile
+- linux.system.shell
 {%- if system.login_defs is defined %}
 - linux.system.login_defs
 {%- endif %}