Hi, guys.

Recently we talked to Leonid in chat and discussed the necessity of
removing flags. I've offered Leonid to check behavior, so here is what
I've found.

My point of view (now and then) is that they're neither useful nor
harmful. Earlier I did agree to blacklist them, but looking at Leonids

Inline help says:

```
--lua-dir=<prefix>     Which Lua installation to use.
--lua-version=<ver>    Which Lua version to use.
--global               Use the system tree when `local_by_default` is `true`.
--local                Use the tree in the user's home directory.
                       To enable it, see '/opt/tarantool-install/bin/tarantoolctl help path'.

Rocks trees in use:                  
        /tmp/tmp.JY1fgy8La9/.rocks ("user")
        /opt/tarantool-install ("system")    
```

* Tarantool does define `LOCAL_BY_DEFAULT = true`,
  (i.e. `--local` is our default behavior)

* "System tree" is the place where tarantool is installed
  (`-DCMAKE_INSTALL_PREFIX`).

* "Home directory" in tarantool is overriden with `./.rocks`.

* See "rocks: update luarocks to 3.1.1" 4222c1f6.

* Both `--local` and `--global` are only shortcuts for predefined
  `--tree=<tree>`, which isn't going to be removed.

* Speaking of `--lua-dir` and `--lua-version`, they look to be neither
  useful nor harmful.

Please, notice that command-line help refers to `tarantoolctl help
path`, but

1. It should be `tarantoolctl rocks help` (I guess there are many of
   such places)
2. `rocks path` command is disabled in your patch.

I'm a lazy developer and I don't like unnecessary restrictions. If I
were you, I'd drop the second commit ("Delete flags which can't be used
with tarantoolctl rocks" 1b51b2f) and enable back `rocks path` command.
Let people decide whether they need it or not.

Finally, a few words about documentation. I've tried to write down all
supported commands and flags, and I found it's really difficult and
verbose. I'm withdrawing my suggestion to clarify it, I don't know what
to do. Perhaps, documentation should just refer to inline help, as it's the
only up-to-date information, and still correct enough.