Refactor auth by agency

[zengchen1024]

No need to add header of "X-Auth-Token" actively, because the provlider client will add it automaticly at here if client.TokenID is not empty.

[coveralls]

Pull Request Test Coverage Report for Build 379

• 0 of 52 (0.0%) changed or added relevant lines in 2 files are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 68.202%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
provider_client.go	0	3	0.0%
openstack/client.go	0	49	0.0%

Files with Coverage Reduction	New Missed Lines	%
openstack/client.go	5	17.37%

Totals
Change from base Build 377:	-0.2%
Covered Lines:	11876
Relevant Lines:	17413

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 384

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 68.202%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
openstack/client.go	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
openstack/client.go	1	17.37%

Totals
Change from base Build 381:	0.004%
Covered Lines:	11876
Relevant Lines:	17413

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💛 - Coveralls