Proxy support

[abpetrov]

I am trying to use the plugin behind a proxy, but it fails with the message below:
* 1 error occurred: * atlas client retrieval: Post "https://emea.api.hvca.globalsign.com:8443/v2/login": dial tcp: lookup emea.api.hvca.globalsign.com on 10.0.0.33: no such host
Could you add support for http_proxy and https_proxy environment variables, see https://developer.hashicorp.com/vault/docs/plugins/plugin-management for details.