fixes #2249: sanitize tenant and tenantid

[GarageDeveloper]

If tenant or tenantid are passed as env variables, we systematically use Sprint to make sure they are string and not integer as it would make mapstructure fail.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master" git@github.com:GarageDeveloper/distribution.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

Merging #2471 into master will decrease coverage by 9.85%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2471      +/-   ##
==========================================
- Coverage   61.42%   51.57%   -9.86%     
==========================================
  Files         128      128              
  Lines       11685    11691       +6     
==========================================
- Hits         7178     6030    -1148     
- Misses       3615     4909    +1294     
+ Partials      892      752     -140

Impacted Files	Coverage Δ
registry/storage/driver/swift/swift.go	57.77% <0%> (-0.74%)	⬇️
registry/storage/driver/gcs/gcs.go	0.4% <0%> (-68.94%)	⬇️
registry/storage/driver/oss/oss.go	0.56% <0%> (-57.23%)	⬇️
registry/storage/driver/s3-aws/s3.go	4.57% <0%> (-56.23%)	⬇️
registry/storage/driver/s3-goamz/s3.go	0.5% <0%> (-51.4%)	⬇️
registry/client/transport/transport.go	69.69% <0%> (-9.1%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f411848...8777e97. Read the comment docs.

[stevvooe]

LGTM