When multiple instances of beem was run in parallel, following error appeared
from time to time (the detailed content of error is attached at the very bottom):
```
>           mkdir(name, mode)
E           FileExistsError: [Errno 17] File exists: '/root/.local/share/beem'
/usr/lib/python3.8/os.py:223: FileExistsError
```

I assume that problem occured, because two processes at the same time checked
if directory exists. Both received same answer -- directory doesn't exists.
So first successfully created the directory, and second one failed with this
error.

Solution implemented in this commit, is to accept situation, when directory
already exists.

Detailed error description:
```
/builds/hive/hive/venv/lib/python3.8/site-packages/beem-0.24.22-py3.8.egg/beem/blockchaininstance.py:186: in __init__
    self.config = kwargs.get("config_store", get_default_config_store(**kwargs))
/builds/hive/hive/venv/lib/python3.8/site-packages/beem-0.24.22-py3.8.egg/beem/storage.py:46: in get_default_config_store
    return generate_config_store(SqliteConfigurationStore, blockchain="hive")(*args, **kwargs)
/builds/hive/hive/venv/lib/python3.8/site-packages/beem-0.24.22-py3.8.egg/beemstorage/sqlite.py:198: in __init__
    SQLiteFile.__init__(self, *args, **kwargs)
/builds/hive/hive/venv/lib/python3.8/site-packages/beem-0.24.22-py3.8.egg/beemstorage/sqlite.py:59: in __init__
    os.makedirs(self.data_dir)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
name = '/root/.local/share/beem', mode = 511, exist_ok = False
    def makedirs(name, mode=0o777, exist_ok=False):
        """makedirs(name [, mode=0o777][, exist_ok=False])

        Super-mkdir; create a leaf directory and all intermediate ones.  Works like
        mkdir, except that any intermediate path segment (not just the rightmost)
        will be created if it does not exist. If the target directory already
        exists, raise an OSError if exist_ok is False. Otherwise no exception is
        raised.  This is recursive.

        """
        head, tail = path.split(name)
        if not tail:
            head, tail = path.split(head)
        if head and tail and not path.exists(head):
            try:
                makedirs(head, exist_ok=exist_ok)
            except FileExistsError:
                # Defeats race condition when another thread created the path
                pass
            cdir = curdir
            if isinstance(tail, bytes):
                cdir = bytes(curdir, 'ASCII')
            if tail == cdir:           # xxx/newdir/. exists if xxx/newdir exists
                return
        try:
>           mkdir(name, mode)
E           FileExistsError: [Errno 17] File exists: '/root/.local/share/beem'
/usr/lib/python3.8/os.py:223: FileExistsError
```