Bug 924833 - Re-enable test_receive_call.py

Add a step to check that there are credits available for making a call
Add test to manifest

Address code review comments:
- Fail when Plivo balance is insufficient
- Break out arguments to PlivoUtil __init__
- Use marionette.Wait for waiting
- Add a custom PlivoError exception
- General code cleanup

Address pull request comments:
- move check for balance into make_call in the utility class
- address syntax nits
- add custom message to custom exception
- change syntax for custom exceptions