Default configuration for exec

That would be great to have an ability to specify default configuration for `exec` in kapacitor.conf:
```
[exec]
enabled = true
exec = /path/to/script arg1 arg2
global = true
state-changes-only = true
```
This would allow setting:
1) default action for all alerts (exec);
2) default configuration for exec (no need to specify `.stateChangesOnly`, path, arguments in every alert).

For now it doesn't seem to work.

debian 9 amd64
kapacitor 1.4.1

Start with the [exec] section proposed for kapacitor.conf and trace how exec settings are read and applied to alerts. Confirm how the global default action and default path, arguments, and state-changes-only behavior should interact with per-alert settings; done means valid defaults work without repeating those values in every alert.

Written by the indexing model from the issue text.