Support setting properties for storage_root_path

[vagetablechicken]

ref #2195

We can specify the properties of storage_root_path by setting ':', seperate by ','
e.g.
storage_root_path = /home/disk1/palo,medium:ssd,capacity:50

[vagetablechicken]

property medium has a higher priority, so I decide to set storage_medium in c'tor of DataDir, change
'_init_extension_and_capacity()' to '_init_capacity()'.
https://github.com/apache/incubator-doris/blob/14769b0beb7671626da011611d2c73ffe841b831/be/src/olap/data_dir.h#L132

But in meta_tool,
https://github.com/apache/incubator-doris/blob/14769b0beb7671626da011611d2c73ffe841b831/be/src/tools/meta_tool.cpp#L143
var 'data_dir' will not be set the _storage_medium, cause I remove the _init_extension part.

I don't want to redefine TStorageMedium enum, e.g. {UNSET, HDD,SSD}.
How about

    DataDir(const std::string& path,
            int64_t capacity_bytes = -1,
            std::string medium_property = "",
            TabletManager* tablet_manager = nullptr,
            TxnManager* txn_manager = nullptr);

Thus,

1. medium_property != "", set _storage_medium
2. medium_property == "", use extension

[imay]

@vagetablechicken

1. I think meta tool should not care about the medium of data path.
2. Even if meta tool care about medium, it is better to make medium and capacity parse in one place. You can make config parse a function, which can be called by both options and meta dir.

[imay]

Good Job
LGTM