Adding proxy user creation per job execution

Changes
* adding sahara.utils.proxy module
* adding functions to create and delete proxy users
* adding proxy user creation during job execution
* adding proxy user deletion during job execution status update
* adding sanitization for proxy configs from job execution
* adding unit test for proxy usage detection function

Partial-implements: blueprint edp-swift-trust-authentication
Change-Id: I551a14a3cb5320a27fc6104b35c3b9a08a03abda
Merge "Adding proxy user creation per job execution"