This commit fixes the Manta gateway client creation flow. We now affix
the endpoint scheme with endpoint URL while creating the Manta client
for gateway.
Also add steps in Manta gateway docs on how to run with custom Manta
endpoint.
Fixes#6408
Manta has the ability to allow users to authenticate with a
username other than the main account. We want to expose
this functionality to minio manta gateway.
Manta is an Object Storage by [Joyent](https://www.joyent.com/)
This PR adds initial support for Manta. It is intended as non-production
ready so that feedback can be obtained.
