support credential export/import

[immarvin]

this PR is for feature request #66:

1. add a new object type credential
2. the credential object contain the xcatd SSL CA certificate, private key, root's client SSL credential and server side SSL certificate

[root@c910f03c05k21 inventory]# xcat-inventory export -t credential
{
    "credential": {
        "credential": {
            "CA": {
                "certificate": "/etc/xcat/ca/ca-cert.pem",
                "private_key": "/etc/xcat/ca/private/ca-key.pem"
            },
            "client": {
                "root": "/root/.xcat/client-cred.pem"
            },
            "server": "/etc/xcat/cert/server-cred.pem"
        }
    },
    "schema_version": "1.0"
}
#Version 2.14.3 (git commit d74f1eed53ef385d80a9013d46c2a7cff1de06e4, built Wed Jul 18 06:15:52 EDT 2018)

3. xcat-inventory export -d can be used to export the definition file and the credential files to destination directory
4. xcat-inventory import -d is used to import the credential files to the correct place

UT:

export:

[root@c910f03c05k21 inventory]# xcat-inventory export -t credential -d /tmp/credential/
The credential objects has been exported to directory /tmp/credential/
[root@c910f03c05k21 inventory]# tree -a /tmp/credential/
/tmp/credential/
└── credential
    ├── definition.json
    ├── etc
    │   └── xcat
    │       ├── ca
    │       │   ├── ca-cert.pem
    │       │   └── private
    │       │       └── ca-key.pem
    │       └── cert
    │           └── server-cred.pem
    └── root
        └── .xcat
            └── client-key.pem

8 directories, 5 files

import:

[root@c910f03c05k21 inventory]# xcat-inventory import  -d /tmp/credential/
Inventory import successfully!
Warning: the /etc/xcat/ca/ca-cert.pem already exists, will be overwritten
Warning: the /etc/xcat/ca/private/ca-key.pem already exists, will be overwritten
Warning: the /root/.xcat/client-key.pem already exists, will be overwritten
Warning: the /etc/xcat/cert/server-cred.pem already exists, will be overwritten
The object credential has been imported