Add HTTP proxy support to ceilometer client

There is a need for HTTP Proxy support in Ceilometer client
It's used in the case when Ceilometer API is not directly available
Proxy url is taken from environment variables http_proxy or https_proxy

Implements: blueprint proxy-support

Change-Id: I839d704a7fa16521292205166d7cbea56497842b
sync with oslo-incubator

- py3 compatibility fixes
- Only pass non-None sortby to PrettyTable.get_string fix

synced with commit-id: 0d2546b228f13a42355bc54e82256bf17a2eb478

Change-Id: Ic70eba1a10339ad5407541446cd37c9229836965
Merge "Add HTTP proxy support to ceilometer client"