Support multi-PodManager

This BP's goal is implementing dynamic management for PodManager in valence platform.
Now pod manager is locked in the valence.conf , this BP would make it more scalable.

To implement this BP , we may need to add a database table 'pod_manager' to recode every pod managers .

Table pod_manager schema maybe :

                     created_at | datetime
                     updated_at | datetime
                     id | int(11)
                     name | varchar
                     ipaddress | varchar
                     username | varchar
                     password | varchar
                     url | varchar
                     status | varchar
                     description | varchar

And we would provide these functions or apis for pod manager management :

/api/pod_manager :
         GET : get pod manager info with a param : <pod_manager_id>
         PUT : update pod manager properties with body : <json:update_items>
         POST : add a new pod_manager in the platform with body : <json:create_items>
         DELETE : delete a exist pod_manager with a param : <pod_manager_id>

Also this change will effect other resource management featurs as follows :

        a. components listing would able change to get_<resource_list>_by_pod_manager
        b. composed operation also need to specify a exist pod_manager to do the operation
        c. Other changes after our discussions

Spec-doc : https://github.com/mshubian/3D_DataCenter/wiki/Valence-support-Muti-PodManager