Allow system calls to pull global settings from the database.

This adjusts the System() class to first try local settings of the
supplied value, fall back to global settings, and then the supplied
default, before finally erroring. This corrects an issue where the
VideoGrabber() child class did not properly pull the global settings for
TelevisionGrabber and MovieGrabber, and thus just used the internal
defaults.
(cherry picked from commit 4e072b2de29c32c4d00fe62849feadd80d2c0c02)
Change VideoGrabber() movie default to tmdb3.py.
Force use of tmdb3.py when user tries to use tmdb.py.

Since the tmdb.py grabber using the old v2.1 API is no longer
functional, and we cannot issue a schema update to force users on the
-fixes branches to transition to it, this detects when users attempt to
run that grabber, and automatically replace it with the new grabber.
Remove tmdb.py and associated library.
(cherry picked from commit 53c272dc4ffe2f5392ff54fb3e446801395274fb)

Conflicts:

	mythtv/programs/scripts/metadata/Movie/tmdb.py
Remove tmdb library from python installer script.
(cherry picked from commit 9c0dfbc9f6b753a5b3d6f2e294baaf9577519a91)
Remove more tmdb bits.
(cherry picked from commit a5d54217962a706c4cedc33012e9e7049d4877f6)