nuageinit: Create parent directories in write_files

Currently, 'write_files' does not create parent directories, and
'runcmd' cannot be used here, since those scripts run after the files
have been written. The only workaround is to create the files in an
existing directory, such as '/root' or '/tmp', and then move those
files using 'runcmd', but this is cumbersome when there are many files,
even if they are small.

With this change, nuageinit now creates the parent directories for each
file using the path field, which mimics the same behavior as in
cloud-init.

Permissions and ownership can also be configured using 'runcmd'.

Reviewed by: bapt@
Approved by: bapt@
Differential Revision: https://reviews.freebsd.org/D57395