[1LP][RFR] Fix typo in skip_msg format when provider setup fails.

[tpapaioa]

This PR makes a small fix to the construction of the skip_msg string passed to pytest.skip() when the setup_provider fixture fails to set up the provider. The skip_msg variable was accidentally set to be a tuple instead of a string, resulting in a TypeError:

E       TypeError: Skipped expected string as 'msg' parameter, got 'tuple' instead.
E       Perhaps you meant to use a mark?

cfme/fixtures/provider.py:87: TypeError
TypeError
b"Skipped expected string as 'msg' parameter, got 'tuple' instead.\nPerhaps you meant to use a mark?"

[john-dupuy]

Thanks @tpapaioa!