Cant extend array or object configuration via environment variables

Currently users can only modify existing configuration elements.

The following test will panic:
```
func TestConfig_Parse_EnvOverrideAddArray(t *testing.T) {
// Parse configuration.
var c server.Config
if _, err := toml.Decode(`
[[influxdb]]
urls=["http://localhost:8086"]
`, &c); err != nil {
t.Fatal(err)
}

if err := os.Setenv("KAPACITOR_INFLUXDB_0_URLS_0", "http://localhost:18086"); err != nil {
t.Fatalf("failed to set env var: %v", err)
}

if err := os.Setenv("KAPACITOR_INFLUXDB_1_URLS_0", "http://localhost:7076"); err != nil {
t.Fatalf("failed to set env var: %v", err)
}

if err := c.ApplyEnvOverrides(); err != nil {
t.Fatalf("failed to apply env overrides: %v", err)
}

if c.InfluxDB[0].URLs[0] != "http://localhost:18086" {
t.Fatalf("unexpected url 0: %s", c.InfluxDB[0].URLs[0])
} else if c.InfluxDB[1].URLs[0] != "http://localhost:7076" {
t.Fatalf("unexpected url 0: %s", c.InfluxDB[0].URLs[0])
}
}
```

Users should be able to add additional configuration to Kapacitor through environment variables.

Start by locating Config.ApplyEnvOverrides and the configuration parsing tests, then reproduce the behavior with TestConfig_Parse_EnvOverrideAddArray. Trace how KAPACITOR_INFLUXDB_0_URLS_0 and KAPACITOR_INFLUXDB_1_URLS_0 are handled; done means the test no longer panics and both URLs are available in c.InfluxDB with the expected values.

Written by the indexing model from the issue text.