generate: Set ociVersion if it wasn't set in the template
generate: Set ociVersion if it wasn't set in the template

The default template has this information already, but user-supplied
templates may not.  Before this commit:

  $ ocitools generate --template <(echo '{}')
  $ head -n2 config.json
  {
    "ociVersion": "",

And after this commit, the last line is:

    "ociVersion": "0.6.0-dev",

Regardless of this commit, template authors can set whatever version
they like without getting clobbered:

  $ ocitools generate --template <(echo '{"ociVersion": "xyz"}')
  $ head -n2 config.json
  {
    "ociVersion": "xyz",

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

hmeng-19/ocitools,opencontainers/ocitools,opencontainers/runtime-tools,wking/ocitools-v2,opencontainers/runtime-tools,liangchenye/ocitools,wking/ocitools-v2,opencontainers/ocitools,liangchenye/ocitools,hmeng-19/ocitools