GUACAMOLE-1881: Add parameter token containing the domain of LDAP/AD usernames.

[mike-jumper]

This change is intended to supersede the changes proposed by poor @josnabattula via PR #931 that I have been very slow to review. It builds off the original commit (27bbd35a3d8d7bfb41ac50dfc145e1314121d289) by:

• Removing unnecessary use of TokenName.canonicalize() (there's no need to dynamically derive the token name of a static value).
• Decoupling the concept of extracting the domain from the concept of using that domain to create a token (getDomainToken() renamed and redocumented as getUserDomain()).
• Removing unnecessary recompilation of the domain extraction Pattern (there's no need to repeatedly recompile the regex each time a user logs in - it never changes).

[josnabattula]

@mike-jumper Can we get these changes into next release, its been months since I have raise pull request.

[josnabattula]

@mike-jumper any plans to look into this pull request? it's been more than 6 months.

[mike-jumper]

@josnabattula Are you able to see Nick's comments above?

[josnabattula]

@josnabattula Are you able to see Nick's comments above?

I can see. But those comments are on your branch you have refactored after my commit cheery-picked. I am thinking you are going to resolve it.

[mike-jumper]

Oof, right - that was silly of me. Sorry about that, @josnabattula.

[josnabattula]

@necouchman Could you review again? All review comments are resolved now. thanks.