make api_extensions_path relative to pybasedir

Routes would fail to load otherwise and some trove commands will 404.

See https://bugs.launchpad.net/trove/+bug/1316195 for more

Change-Id: Idac8c3d899fbe9da3a10faf25d38179adc016e9a
Merge "make api_extensions_path relative to pybasedir"