Fixed scope override inside metafunc.parametrize. Fixes #634

[Stranger6667]

Hello, this is my initial approach to solve #634
I'm not very familiar with pytest codebase, so I'll appreciate any help here.
Thank you

P.S. Also #684 #661 #896 #531 are probably related to this issue

[coveralls]

Coverage increased (+0.004%) to 92.296% when pulling af362b3 on Stranger6667:issue634-scopes into d37af20 on pytest-dev:master.

[coveralls]

Coverage increased (+0.004%) to 92.296% when pulling e686f39 on Stranger6667:issue634-scopes into d37af20 on pytest-dev:master.

[RonnyPfannschmidt]

@Stranger6667 good work, at first glance it indeed seems to solve quite many if not all of the related issues

[coveralls]

Coverage increased (+0.004%) to 92.301% when pulling d72afe7 on Stranger6667:issue634-scopes into a2b04d0 on pytest-dev:master.

[nicoddemus]

Thanks!