leap vm add should remove node definition file on failure

If leap vm add fails, a node definition file will remain, with possible wrong content, making subsequent cmds fail as well:

--- ci/ci-provider ‹master* M› » leap vm add --wait mynode tags:ci
 = created nodes/mynode.json
 = error in node `mynode`: the tag "ci" does not exist!

 = error in node `mynode`: error in node `mynode`: the tag "ci" does not exist!

--- ci/ci-provider ‹master* M⁇› » leap vm add 
 = error in node `mynode`: the tag "ci" does not exist!


 = error NoMethodError: undefined method `each' for nil:NilClass

(from redmine: created on 2016-09-07, relates #8450 (closed))