Allow to specify default partition sizes

[mudler]

Is your feature request related to a problem? Please describe.
At the moment we hardcode the partition size in cos-installer and more importantly we have hardcoded image size during upgrade/reset. This might work for all the cases, but makes sense to expose this to the configuration since it might not suit all cases.

Describe the solution you'd like
A way to override default configs with our general config file, or alternatively via cloud-init:

• Partition sizes
• Partition types (at least should be possible to specify ext4)
• Default image size (.img files created during upgrades)
• Default COS_PERSISTENT device location

Describe alternatives you've considered
N/A

Additional context
Hardcoded image size:
https://github.com/rancher-sandbox/cOS-toolkit/blob/master/packages/installer/upgrade.sh#L113
Hardcoded partition size:
https://github.com/rancher-sandbox/cOS-toolkit/blob/master/packages/installer/installer.sh#L111

[mudler]

Depends on #771

[mudler]

As a first pass, makes sense to explore if we can do the partitioning with the cloud-init tool, so:

• We have one tool only to handle partitioning logics
• It is tested - the cloud-init tool layout plugin have unit tests and can be easily maintained
• We can allow the user to provide optionally a cloud-config file with the installation partitioning schema

In this way, we don't have to re-implement any partitioning logic into the bash CLI, but we can just generate a cloud-config on-the-fly during installation (or just ship sane defaults), if the user didn't provided one