Enable overwrite for timeline, report, trace, and dag (#325)

potential fix for gcp error
```
 {                                                                                                                                                                              "error": {                                                                                                                                                                     "code": 412,                                                                                                                                                                 "message": "At least one of the pre-conditions you specified did not hold.",                                                                                                 "errors": [                                                                                                                                                                    {                                                                                                                                                                              "message": "At least one of the pre-conditions you specified did not hold.",                                                                                                 "domain": "global",                                                                                                                                                          "reason": "conditionNotMet",                                                                                                                                                 "locationType": "header",                                                                                                                                                    "location": "If-Match"                                                                                                                                                     }                                                                                                                                                                          ]                                                                                                                                                                          }                                                                                                                                                                          }
```

Author: matthdsm

nextflow.config

@@ -276,18 +276,22 @@ nextflow.enable.configProcessNamesValidation = false
 
 timeline {
     enabled = true
+    overwrite = true
     file    = "${params.outdir}/pipeline_info/execution_timeline_${params.trace_report_suffix}.html"
 }
 report {
     enabled = true
+    overwrite = true
     file    = "${params.outdir}/pipeline_info/execution_report_${params.trace_report_suffix}.html"
 }
 trace {
     enabled = true
+    overwrite = true
     file    = "${params.outdir}/pipeline_info/execution_trace_${params.trace_report_suffix}.txt"
 }
 dag {
     enabled = true
+    overwrite = true
     file    = "${params.outdir}/pipeline_info/pipeline_dag_${params.trace_report_suffix}.html"
 }