health/provider: python: warning with correct host prog (#8049)

I have `g:python3_host_prog` set to the system Python, where a package
is also installed to provide the "neovim" module.

`:checkhealth provider` however displays a warning for this:

> Your virtualenv is not set up optimally.

This is because /usr/bin/python is not in /home/user/.pyenv.

I think this warning should not get displayed if host_prog_var exists.

It goes back to the initial commit (20447ba09), and is maybe only
missing the `!` there as with the previous commit.

Full output:
```
  - INFO: pyenv: /home/user/.pyenv/libexec/pyenv
  - INFO: pyenv root: /home/user/.pyenv
  - INFO: Using: g:python3_host_prog = "/usr/bin/python"
  - WARNING: Your virtualenv is not set up optimally (/usr/bin/python is not in /home/user/.pyenv).
    - ADVICE:
      - Create a virtualenv specifically for Neovim and use `g:python3_host_prog`.  This will avoid the need to install Neovim's Python module in each virtualenv.
  - WARNING: $VIRTUAL_ENV exists but appears to be inactive. This could lead to unexpected results.
    - ADVICE:
      - If you are using Zsh, see: http://vi.stackexchange.com/a/7654
  - INFO: Executable: /usr/bin/python
  - INFO: Python3 version: 3.6.4
  - INFO: python-neovim version: 0.2.1
  - OK: Latest python-neovim is installed: 0.2.1
```

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