Support custom environment name

[shandilya3]

We have made changes to support custom environment name as in the Target UI under Administration > Environments.
The default environment is set to production.

For CDN, we are always pointing to assets.adobetarget.com, aligning with our Java SDK implementation.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

coverage: 90.213% (-2.4%) from 92.653%
when pulling 616868e on shandilya3:TNT-47592-environmentFix
into c7aa8ef on adobe:main.

[dcottingham]

Nice work. Just a couple minor comments and then feel free to merge.