Add docs on how to disable cache provider

[nicoddemus]

After discussion in #1029

I suggest we start to advise people who are having problems related to cache to use this trick so it doesn't affect their use of pytest-2.8 and force them to pin down to pytest-2.7.X.

[RonnyPfannschmidt]

can it actually be reenabled?

[nicoddemus]

Do you mean without removing the option from pytest.ini manually? I don't know the order of precedence in this cases, but I would guess command line wins configuration.

[hpk42]

i wonder if it's the right thing to advise to do this on a per-project level. You might want cache behaviour on dev machines but not on CI systems. Therefore we should rather advise for setting the environment variable PYTEST_ADDOPTS for respective environments.

[hpk42]

in addition we should see to add some info about environment variables with py.test --help i guess. (like "tox -h" does) (but than can be done in a separate future PR).

[nicoddemus]

Added environment variables to --help as requested, and moved plugin disabling to plugins.txt. I think the more detailed explanation fits there better.

[nicoddemus]

Can we merge this?

[RonnyPfannschmidt]

of course, sorry for the delay

[nicoddemus]

No worries! 😄