Improvements to snap refresh related to nvidia support

[jocado]

This addresses two current known issues to snap refresh if nvidia support is in use.

The previous CDI config is never valid for a newer revision, as some paths contain the snap revision.

• This PR ensures the previous config is always purged

If the nvidia-container-toolkit service fails to start properly for some reason, it prevents the snap refresh from completing, even though dockerd would remain functional for non nvidia images use cases.

• This PR enables nvidia-container-toolkit to fail setting up the CDI and dockerd config, and print a warning instead.
• It also ensures that dockerd config is only attempted if CDI config is generated.
• CDI config can fail on refresh due to: ERROR_LIB_RM_VERSION_MISMATCH [ nvidia kernel module and user space libs do not match ].

[jocado]

@xnox PTAL when you get a chance - thanks.