Use bundled version of PTVSD in debugger

[DonJayamanne]

Today the debugger ends up using the version of PTSVD installed in the current environment (available in the current path) as opposed to using what's bundled in the extension.

We might want to ensure we always use the bundled version of PTVSD and allow users to configure whether to use bundled version or their own copy of PTVSD.

[DonJayamanne]

Suggestion:

• Add a setting customDebugger into launch.json settings.
• If this is initialized to true, then we use PTVSD that's in the current path. E.g. if PTVSD is installed in current python environment,
• Else, we always use PTVSD that's bundled with the extension.

[DonJayamanne]

Why had this been reopened?

[brettcannon]

@DonJayamanne I don't remember seeing this go through the PM sign-off stage for the new flag.