This commit allows users to access a vCenter or a ESXi through a
HTTP CONNECT based proxy.
To do so, the users have to set the `proxy_host` and `proxy_port`
variables.
The can also use the `VMWARE_PROXY_HOST` and `VMWARE_PROXY_PORT`
environment variables.
This feature depends on pyvmomi > v6.7.1.2018.12.
Fixes: #42221
Co-Author: Abhijeet Kasurde <akasurde@redhat.com>
Co-Author: Gonéri Le Bouder <goneri@redhat.com>
