Chronograf: Add "connect to" influxdb and/or kapacitor from CLI

[goller]

From #318

[nhaugo]

• If there are no sources, add CLI source and make it the default
• If there is a default add the source as non default and connect to it instead the default

[lukevmorris]

After discussing UX, proposed new CLI options:

  --influxdb=http://localhost:8086
  --influxdb-username=admin
  --influxdb-password=password

  --kapacitor=http://localhost:9092
  --kapacitor-username=admin
  --kapacitor-password=password

Also available in ENV_VAR format:

export INFLUXDB=http://localhost:8086
export INFLUXDB_USERNAME=admin
export INFLUXDB_PASSWORD=password

export KAPACITOR=http://localhost:9092
export KAPACITOR_USERNAME=admin
export KAPACITOR_PASSWORD=password