api/uniter: validate unit name before relation-get

The name of the unit passed in from the cli is not a string for of a tag so we have to validate that it actaully is a unit name (the id part of a unit tag).
Merge pull request #677 from davecheney/194-api-uniter-validate-unit-name

api/uniter: validate unit name before relation-get

The name of the unit passed in from the cli is not a string form of a tag so we have to validate that it actaully is a unit name (the id part of a unit tag).