CentOS finalize curthook duplicates MAAS credentials

[Lee Trager]

Both curt-hooks and finalize are creating MAAS credentials in /etc/cloud/cloud.cfg.d. curt-hooks creates 50-cloudconfig-maas-reporting.cfg from the cloudconfig section of the Curtin config. finalize is creating 90_datasource.cfg using the debconfig_selections from the Curtin config. 90_datasource.cfg is only needed when CentOS is deployed with old versions of MAAS/Curtin(from Trusty).

This should be documented in finalize.py and if CENTOS_CURTHOOK_SUPPORT is in the Curtin feature list finalize.py should not be run.

[Ryan Harper (raharper)]

FYI, when testing curtin SRU (https://bugs.launchpad.net/ubuntu/+source/curtin/+bug/1795712) on MAAS Bionic/2.4 we found that centos7 images fail to report back to maas and don't have a datasource.

I believe this is due to the change on this branch which detected the curtin CENTOS_CURTHOOK_SUPPORT but didn't confirm that MAAS also sent a storage config (aka MAAS 2.5+) and so finalize fails to write out a 90_datasource.cfg file.

This change needs to also check if cfg includes the 'storage' key before skipping generation of 90_datasource.cfg (like how the check is done in contrib/centos/centos7/curtin/curtin-hooks.py:

 # Run builtin hooks if curtin has the feature and maas sent storage cfg
 if centos_curthooks and cfg.get('storage'):