Fixes bug when pulling an official image (no user namespace) with a specified tag
Merge pull request #543 from dotcloud/pull-official-tag-fix

- Registry: Fix pull for official images with specific tag
kernel version - don't show the dash if flavor is empty
Check the command existance prior create and add Unit tests for the case
Merge pull request #552 from dotcloud/548-no_command_panic-fix

- Builder: Check the command existance prior create and add Unit tests for the case
Merge pull request #550 from unclejack/handle-empty-kernel-flavor-without-dash

* Runtime: kernel version - don't show the dash if flavor is empty