MAGN-5593: Use local reference of RevitDynamoModel in RevitVisualizationManager

## Issue

http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-5593

Essentially, a `NullReferenceException` was being thrown from line 69 of DynamoRevit.cs as the `revitDynamoModel` had been set to null.

## Stop-gap

Simply provide a valid reference of `RevitDynamoModel` as one of the arguments to `ExecuteOnIdleAsync`.  All usages of this method are also updated.

## Merge

- [ ] Revit2015
- [ ] 0.7.5RC
- [ ] 0.7.5RC2015