KNOX-2714 - Added doAs support to KnoxToken service

[smolnar82]

What changes were proposed in this pull request?

Implemented the changes described in KNOX-2714.

How was this patch tested?

Updated existing uni tests as well as added new ones to cover the new functionality.

Manual testing is done:

1. Logged in as the admin user
2. Created a token for the admin user itself
3. Created tokens for different machine users: sandormolnar and tom

postgres=> select * from knox_token_metadata;
               token_id               |  md_name  |                                         md_value                                         
--------------------------------------+-----------+------------------------------------------------------------------------------------------
 8ceb8731-5b73-4983-9c9d-ff2712c76daa | comment   | token for admin
 8ceb8731-5b73-4983-9c9d-ff2712c76daa | userName  | admin
 8ceb8731-5b73-4983-9c9d-ff2712c76daa | enabled   | true
 8ceb8731-5b73-4983-9c9d-ff2712c76daa | passcode  | emzvv73vv71Ie++/ve+/ve+/vQHvv70kaWwLJu+/ve+/vXjRmu+/vdG1OR/vv71b77+977+977+9LA==

 80ce72a3-221d-49e6-971d-d4d4886d29d0 | comment   | token for sandormolnar
 80ce72a3-221d-49e6-971d-d4d4886d29d0 | userName  | sandormolnar
 80ce72a3-221d-49e6-971d-d4d4886d29d0 | createdBy | admin
 80ce72a3-221d-49e6-971d-d4d4886d29d0 | enabled   | true
 80ce72a3-221d-49e6-971d-d4d4886d29d0 | passcode  | 77+9Ne+/vRdkYwzElXgbRe+/ve+/ve+/vWNUUO+/vSsfNO+/vU/vv71k77+9O3jvv73vv710

 8e17c9d5-ca59-4672-802f-f6c20287deec | comment   | token for tom in accountant team
 8e17c9d5-ca59-4672-802f-f6c20287deec | userName  | tom
 8e17c9d5-ca59-4672-802f-f6c20287deec | createdBy | admin
 8e17c9d5-ca59-4672-802f-f6c20287deec | enabled   | true
 8e17c9d5-ca59-4672-802f-f6c20287deec | passcode  | 77+977+977+9ce+/vRPvv70+Iu+/vXJl77+9ce+/vSrvv71777+977+977+977+9Mcu6M3nvv73vv71eKu+/vQ==
(14 rows)

4. Removed knox.token.proxyuser.admin.users and changed knox.token.proxyuser.admin.groups to accountant in the homepage topology (by default 'admin' is allowed to impersonate everyone). Then I tried to create a token on behalf of bob who is not in the accountant group (he's a researcher):

$ id bob
uid=505(bob) gid=20(staff) groups=20(staff),504(research)
The result, as expected, is 403

[pzampino]

LGTM