dvc: do not set cache dir if it's not a dvc repo

[skshetry]

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Fixes #3328

[skshetry]

Let's fail with the test for now. After it fails, I'll fix a push in another commit.

[skshetry]

Failed nicely. CI log:

=================================== FAILURES ===================================
_______________ test_config_loads_without_error_for_non_dvc_repo _______________
[gw3] linux -- Python 3.7.1 /home/travis/virtualenv/python3.7.1/bin/python
tmp_dir = PosixTmpDir('/tmp/pytest-of-travis/pytest-0/popen-gw3/test_config_loads_without_erro0')
    def test_config_loads_without_error_for_non_dvc_repo(tmp_dir):
        # regression testing for https://github.com/iterative/dvc/issues/3328
>       Config(validate=True)
/home/travis/build/iterative/dvc/tests/func/test_config.py:108: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/home/travis/build/iterative/dvc/dvc/config.py:223: in __init__
    self.load(validate=validate)
/home/travis/build/iterative/dvc/dvc/config.py:282: in load
    self["cache"]["dir"] = os.path.join(self.dvc_dir, "cache")
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
a = None, p = ('cache',)
    def join(a, *p):
        """Join two or more pathname components, inserting '/' as needed.
        If any component is an absolute path, all previous path components
        will be discarded.  An empty last part will result in a path that
        ends with a separator."""
>       a = os.fspath(a)
E       TypeError: expected str, bytes or os.PathLike object, not NoneType
/home/travis/virtualenv/python3.7.1/lib/python3.7/posixpath.py:80: TypeError
_______________ test_plain_dvc_command_not_fail_on_non_dvc_repo ________________
[gw1] linux -- Python 3.7.1 /home/travis/virtualenv/python3.7.1/bin/python
tmp_dir = PosixTmpDir('/tmp/pytest-of-travis/pytest-0/popen-gw1/test_plain_dvc_command_not_fai0')
git_dir = PosixTmpDir('/tmp/pytest-of-travis/pytest-0/popen-gw1/git-erepo0')
    def test_plain_dvc_command_not_fail_on_non_dvc_repo(tmp_dir, git_dir):
        # regression test for https://github.com/iterative/dvc/issues/3328
>       assert main([]) == 0
E       assert 254 == 0
E         -254
E         +0

/home/travis/build/iterative/dvc/tests/func/test_cli.py:184: AssertionError