datasource.sys_cfg gets different values in local stage and after.

When a datasource is instantiated it is passed in Init.cfg as the 'sys_cfg'.
But then when 'distro' property is referenced, it has the side effect
of:
            system_config = self._extract_cfg('system')

            if self.datasource is not NULL_DATA_SOURCE:
                self.datasource.distro = self._distro
                self.datasource.sys_cfg = system_config

Here we see the problem demonstrated.

root@c1:~# cat show-syscfg-keys
#!/usr/bin/python3
from cloudinit.stages import _pkl_load;
print(_pkl_load("/var/lib/cloud/instance/obj.pkl").sys_cfg.keys())

root@c1:~# cloud-init clean --logs
root@c1:~# cloud-init init --local
Cloud-init v. 18.3-18-g3cee0bf8-0ubuntu1 running 'init-local' at Thu, 16 Aug 2018 18:08:00 +0000. Up 574.00 seconds.

root@c1:~# python3 show-syscfg-keys
dict_keys(['datasource_list', '_log', 'log_cfgs', 'output', 'users', 'disable_root', 'preserve_hostname', 'cloud_init_modules', 'cloud_config_modules', 'cloud_final_modules', 'def_log_file', 'syslog_fix_perms', 'vendor_data'])

root@c1:~#
root@c1:~#
root@c1:~# cloud-init init >/dev/null 2>&1
Cloud-init v. 18.3-18-g3cee0bf8-0ubuntu1 running 'init' at Thu, 16 Aug 2018 18:08:11 +0000. Up 585.00 seconds.
...

root@c1:~# python3 show-syscfg-keys
dict_keys(['default_user', 'ntp_client', 'paths', 'package_mirrors', 'ssh_svcname', 'network'])

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: cloud-init 18.3-18-g3cee0bf8-0ubuntu1
ProcVersionSignature: Ubuntu 4.17.0-6.7-generic 4.17.9
Uname: Linux 4.17.0-6-generic x86_64
ApportVersion: 2.20.10-0ubuntu7
Architecture: amd64
CloudName: LXD
Date: Thu Aug 16 18:02:03 2018
PackageArchitecture: all
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=C.UTF-8
SourcePackage: cloud-init
UpgradeStatus: No upgrade log present (probably fresh install)