Add Datasource UID

[aborilov]

add methods to support GET /api/datasources/uid/:datasourceUID, DELETE /api/datasources/uid/:datasourceUID API calls

[coveralls]

Coverage decreased (-1.3%) to 46.223% when pulling 1808389 on kubermatic:datasource-uid into 8250fdb on grafana-tools:master.

[aborilov]

i've added integration tests, but haven't run them, cause don't have any ideas how

[ameliagapin]

Any chance that this could get merged? I have a need for the uid property on datasource