cc_write_files: Improve schema.

[aciba90]

Allow text/plain in write_files.encoding.

Proposed Commit Message

cc_write_files: Improve schema.

Allow `text/plain` in write_files.encoding.

Additional Context

Test Steps

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[blackboxsw]

Confirm failure prior to this branch and expected fix after

# BEFORE
2022-05-17 20:35:35,184 - schema.py[WARNING]: Invalid cloud-config provided:
2022-05-17 20:35:35,198 - cc_write_files.py[WARNING]: Unknown encoding type text/plain, assuming text/plain
# After upgrade

root@mon:~# cloud-init schema --system
Valid cloud-config: system userdata
root@mon:~# grep WARN /var/log/cloud-init.log 
root@mon:~#