Need ability to set fixed ID property for cdk-overlay-pane

### Issue type

**I'm submitting a ...** (check one with "x")

* [ ] bug report
* [X] feature request

### Issue description

**Current behavior:**
Currently, the cdk-overlay-pane container has a dynamic ID ('#cdk-overlay-' + number of modals that have been opened). Each time a new modal is opened, the number is incremented (#cdk-overlay-1, #cdk-overlay-2 etc).

**Expected behavior:**
For testing purposes, to have the ability to set a fixed ID or to halt the ID counter. Since the modal is destroyed and removed from the DOM after it closes, maybe it wouldn't be a problem to use the same ID.

**Steps to reproduce:**

**Related code:**

### Other information:

**npm, node, OS, Browser**

**Angular, Nebular**