Support regexp path

[dtaniwaki]

I think supporting regexp paths is very useful when you want to monitor multiple files created in a specific directory.

[dtaniwaki]

I found the following error on the gateway-client side when I was testing this function and there's no output on the gateway side. I'm investigating it...

2019-03-01T17:33:01Z | info  | msg: detected ConfigMap addition. Updating the controller run config. |  config-map:hdfs-gateway-configmap gateway-name:hdfs-gateway gateway-namespace:default
2019-03-01T17:33:01Z | info  | msg: event source |  config-key:tmpdir config-value:"directory: \"/tmp/\"\ntype: \"CREATE\"\npathRegexp: \"[0-9]\\\\.txt\"\naddresses:\n- my-hdfs-namenode-0.my-hdfs-namenode.default.svc.cluster.local:8020\n- my-hdfs-namenode-1.my-hdfs-namenode.default.svc.cluster.local:8020\nhdfsUser: root\nkrbCCacheSecret:\n  name: krb\n  key: krb5cc_0\n# krbKeytabSecret:\n#   name: krb\n#   key: user1.keytab\n# krbUsername: \"user1\"\n# krbRealm: \"MYCOMPANY.COM\"\nkrbConfigConfigMap:\n  name: my-hdfs-krb5-config\n  key: krb5.conf\nkrbServicePrincipalName: hdfs/_HOST" gateway-name:hdfs-gateway gateway-namespace:default hash:3664361829
2019-03-01T17:33:01Z | info  | msg: state of the connection |  gateway-name:hdfs-gateway gateway-namespace:default state:READY
2019-03-01T17:33:01Z | info  | msg: event sources hashes |  current-event-sources-keys:null gateway-name:hdfs-gateway gateway-namespace:default
2019-03-01T17:33:01Z | info  | msg: event sources hashes |  gateway-name:hdfs-gateway gateway-namespace:default updated-event-sources--keys:["3664361829"]
2019-03-01T17:33:01Z | info  | msg: stale event sources |  event-sources:null gateway-name:hdfs-gateway gateway-namespace:default
2019-03-01T17:33:01Z | info  | msg: new event sources |  event-sources:["3664361829"] gateway-name:hdfs-gateway gateway-namespace:default
2019-03-01T17:33:01Z | info  | msg: activating new event source |  event-source-name:tmpdir gateway-name:hdfs-gateway gateway-namespace:default
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x102bfdf]

goroutine 55 [running]:
github.com/argoproj/argo-events/gateways.(*GatewayConfig).startEventSources.func1(0xc0004d2300, 0xc00041ed80)
        /home/vaibhav/go/src/github.com/argoproj/argo-events/gateways/event-sources.go:140 +0xef
created by github.com/argoproj/argo-events/gateways.(*GatewayConfig).startEventSources
        /home/vaibhav/go/src/github.com/argoproj/argo-events/gateways/event-sources.go:123 +0x181

[VaibhavPage]

looks like the valid object is nil which it shouldn't be because ValidateGatewayEventSource returns a non-empty valid object

[dtaniwaki]

@VaibhavPage Looks like my the versions of gateway-client and hdfs-gateway are inconsistent. I confirmed the problem above doesn't happen after I updated the gateway-client image.

Could you review this PR?

[VaibhavPage]

lgtm!!! Nice to see file watcher gateway benefitting from common path regex functionality.