nvme_manager: add support for adjust monitoring interval

Add support for adjust monitoring interval by adding monitorIntervalSec
into nvme_config.json.

Default set interval to 1 second if monitorIntervalSec not exists in
config file.

Example of set monitoring interval to 3 second:

{
    "config": [
        ...
    ],
    "threshold": [
        ...
    ],
    "monitorIntervalSec": 3
}

Tested on Bletchley.

Signed-off-by: Potin Lai <potin.lai@quantatw.com>
Change-Id: Iee11b23bd3166b9a35100d41b93f9f98dc3f355d
diff --git a/nvme_manager.hpp b/nvme_manager.hpp
index 0ee23df..ffdb20a 100644
--- a/nvme_manager.hpp
+++ b/nvme_manager.hpp
@@ -155,6 +155,9 @@
     void read();
 
     std::vector<phosphor::nvme::Nvme::NVMeConfig> getNvmeConfig();
+
+    /** @brief Monitor interval in second  */
+    size_t monitorIntervalSec;
 };
 } // namespace nvme
 } // namespace phosphor