Remove replication of code to get current host

[nikochiko]

The code to fetch the current host URL is repeated in these two places:

• https://github.com/Cloud-CV/evalai-cli/blob/master/evalai/set_host.py#L14
• https://github.com/Cloud-CV/evalai-cli/blob/master/evalai/utils/auth.py#L88-L100

The util method get_host_url can be used in the host command instead.

[ayu023ban]

I would like to work on this issue

[RishabhJain2018]

Sure!