[feature] cexec should grab the mountpoints using the metadata

[mgrzybek]

Hello,

I want to run some commands in a chroot, however my installation configuration uses several mountpoints (such as /boot, /, /var/log…).
At the moment cexec uses only the given block device through environment.

I think that cexec could do something like: if BLOCK_DEVICE is empty, then use https://github.com/tinkerbell/actions/tree/main/rootio/storage to read the metadata and mount everything provided by the Hardware object..

This should keep compatibility with the actual behaviour.

[mgrzybek]

I submitted a PR: #167