respect http(s)_proxy env variable for proxy information

The idea behind this is to allow a user to specify the username/password
and other proxy information in an environment variable instead of
the configuration file. Subscription manager will use the values passed in from
the cli or from the configuration file if present otherwise it will try to use
the http(s)_proxy environment variables. The variables are read in the following
order:

HTTPS_PROXY
https_proxy
HTTP_PROXY
http_proxy

Summary of changes:

    * moved parse_url, ServerUrlParse* exceptions, has_(good|bad)_scheme
      from subscription-manager utils.py
    * updated parse_url to handle username/password
    * Add unit tests for parse_util
    * added get_env_proxy_info utility method to hide details of
      working with the env variables.
make ContentConnection proxy aware.
avoid pollution of env, by mock os.environ

If a test fails before the .pop(), it could leave a bogus os.environ until
something unsets it
(possibly nothing, if the last proxy test fails and doesn't pop it)

May be a good use case for
http://www.voidspace.org.uk/python/mock/patch.html#patch-dict
mocking os.environ makes sense for these unit style tests as well.
move translated strings to subscription-manager

import patch from mock
switch to @patch for consistency
protect against user env bleeding into tests
Merge pull request #107 from candlepin/newproxypass

respect http(s)_proxy env variable for proxy information