text: Title: `[testenv]` `install_command` is also used for provision environment
  Body: From #2427. If I specify a high `minversion` or `requires`, a provision environment is created. Apparently, it is using `install_command` from `[testenv]` section. Is this right? I thought that `testenv` means test environment and will not be used for anything else.
  
  If this is correct behavior, perhaps document it? Also document the way to make provision environment not use test environment. Apparently, this works:
  
  ```ini
  [testenv:.tox]
  install_command = python -m pip install {opts} {packages}
  
  [testenv]
  install_command = your_custom_command
  ```
  
  <details>
  <summary>Moved to discussions</summary>
  
  P.S. Why is it necessary to specify the full env name in a section? As in, why require this: `[testenv:my-cool-env]` but disallow this: `[testenv:cool]` or `[testenv:!.tox]`?
  </details>
  
  <details>
  <summary>Moved to another issue</summary>
  
  Also, documentation for `install_command` says, 
  
  > Note: You can also provide arbitrary commands to the install_command. 
  
  I don't think this is correct? Using this configuration, and running with `-vvvr`, I only see the first install command being executed:
  ```
  [tox]
  skip_install = true
  skipsdist = true
  envlist = foo
  
  [testenv]
  install_command =
      echo '!!!!!!!!!!!!!!!!!!!!!!! install 1' {packages}
      echo '!!!!!!!!!!!!!!!!!!!!!!! install 2' {packages}
  commands =
      echo '!!!!!!!!!!!!!!!!!!!!!!! test' {envname}
  deps =
      zoo
  ```
  
  If `install_command` only accepts one command, perhaps fail if there are many? And the documentation shouldn't say it accepts command**s**?
  
  </details>
  <details>
  <summary>This should probably be elsewhere</summary>
  Finally, I want to note that this is not an error:
  
  ```
  install_command =
      echo '\
      ' {packages}
  ```
  
  And this is also not an error:
  
  ```
  install_command =
      foo: echo '\
      ' {packages}
  ```
  But this is fails:
  
  ```
  install_command =
      foo3: echo '\
      ' {packages}
  ```
  
  > File "/usr/lib/python3.8/shlex.py", line 191, in read_token
  > raise ValueError("No closing quotation")
  > ValueError: No closing quotation
  
  </details>