Redundant instance name subdirectories in var/ structure

Problem

The current tt 2.0+ directory structure creates unnecessary nesting by duplicating instance names inside the var/ directory:

instances.enabled/
└── myapp/                    # Instance name
    └── var/
        ├── lib/
        │   └── myapp/        # ← Redundant duplication
        │       ├── *.snap
        │       └── *.xlog
        ├── log/
        │   └── myapp/        # ← Redundant duplication
        │       ├── tt.log
        │       └── tarantool.log
        └── run/
            └── myapp/        # ← Redundant duplication
                ├── tarantool.pid
                └── tarantool.control

Expected behavior

Since each instance already has its own isolated directory under instances.enabled/, the instance name should not be duplicated inside var/ subdirectories:

instances.enabled/
└── myapp/                    # Instance name (sufficient isolation)
    └── var/
        ├── lib/              # ← Direct data files
        │   ├── *.snap
        │   └── *.xlog
        ├── log/              # ← Direct log files
        │   ├── tt.log
        │   └── tarantool.log
        └── run/              # ← Direct runtime files
            ├── tarantool.pid
            └── tarantool.control

Rationale

1. Redundancy: Instance name is already specified at the top level (instances.enabled/myapp/)
2. Simplicity: Shorter paths are easier to navigate and understand
3. Consistency: Other tools don't typically create nested subdirectories with the same name
4. Migration pain: Current structure makes file paths unnecessarily long and confusing

Configuration impact

This would also simplify box.cfg paths:

Current (redundant):

box.cfg {
    wal_dir = 'var/lib/myapp',
    memtx_dir = 'var/lib/myapp',
    log = 'var/log/myapp/tarantool.log'
}

Proposed (clean):

box.cfg {
    wal_dir = 'var/lib',
    memtx_dir = 'var/lib',
    log = 'var/log/tarantool.log'
}

Environment

• tt version: 2.x+
• OS: Any

Suggested solution

Remove the instance name subdirectories from var/lib/, var/log/, and var/run/ paths, storing files directly in these directories since instance isolation is already provided by the parent directory structure.