FIX: conda config did not use CONDARC environment variable

The `--file` option now defaults to os.environ.get('CONDARC')
Updated default and doc string
Merge branch 'master' into fix/conda-config-rc

Conflicts:
	conda/cli/main_config.py
Add user rc path to conda config --file help
Merge pull request #960 from conda/fix/conda-config-rc

Fixed conda config --file option default