improve initializing ConnectInfo

[krisis]

• Remove ConnectInfo's Default instance
• Add support for reading from well-known credential files and
  environment variables

TBD

• should we move minioPlayCI, awsCI etc to use connectInfo / connectInfoWithCreds?
• should we make then IO ConnectInfo as opposed to pure values?

##TODO

• update documentation
• add examples

[krisis]

This PR passes stack test. I tested with ~/.aws/credentials file containing sample creds and setting relevant env variables. I have posted this for early feedback. If everything looks good I can move examples and docs accordingly.
There are a couple of issues in this new approach

1. It is possible for user to provide a Manager with incorrect ManagerSettings. e.g supplying defaultManagerSettings when endpoint is HTTPS.
2. Too many ways to create ConnectInfo to the point it may confuse new users.

[harshavardhana]

if there is an attempt to do this, I would prefer if you can follow the https://github.com/minio/minio-go/tree/master/pkg/credentials style where we will simply implement many variations of the same credentials.Value structure.

This would allow using minio-hs in a wide range of situations. Maybe not everything but starting with aws creds, iam and environment variables

[donatello]

Link for the module docs: https://godoc.org/github.com/minio/minio-go/pkg/credentials

outdated