[MRG+1] passing apikey as username is suggested by dash

[dangra]

For example in projects like https://staging.scrapinghub.com/p/53/deploy/

# Project: dangra-sandbox
[deploy]
url = https://staging.scrapinghub.com/api/scrapyd/
username = YOUR_LONG_APIKEY_IS_HERE
password =
project = 53

Having a way to pass the apikey in the same section than project and endpoint is quite convenient for Dash development, for i.e.:

[deploy:vagrant]
url = http://vagrant.host.name/api/scrapyd
username = DEVELOPER_APIKEY
project = 1