Support for custom CDN host option

[moritzraho]

Actual behavior

The CDN domain is fixed (see https://github.com/adobe/aio-lib-files/blob/master/lib/impl/AzureBlobFiles.js#L29) and points to the Adobe I/O Firefly CDN, this is a problem if the user is bringing his own storage via Files.init({ azure: <storage creds> }) as URLs would be broken.

Expected behavior

Support for a custom CDN host option on files init, e.g. Files.init({ host: 'mycdn.domain' })
EDIT: Let the TVM return the hostname as part of the credentials, as the logic handling this is already added by #66

Why

• allows support for bring you own storage => DONE in BYO creds related issue #66
• allow to test the files lib against non-prod environments

[moritzraho]

relates to #52

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/ACNA-898

[moritzraho]

Reopening because #66 did not solve the case of using non-prod TVMs, the TVM should return the CDN host name. Which would fix this transparently

[moritzraho]

Closing this stale issue as #90 brought support for our stage TVM.