add support for docker {pull,tag}, and, finally, for app import command

With this PR it is possible to import built-n-ready app container from foreign docker registry:
```bash
$ cocaine-tool app import \
      --container-url=foreign-registry.host/container-name \
      --docker-address=http://localhost:12345 \
      --manifest='{"slave":"/something"}' \
      --registry=cocaine-registry \
      --name=something_blabla_v1518
```
It's going to be useful for migrating containers from testing cluster to production.
Misleading log message - you're not actually checking Dockerfile here.