Adding support for oslo.rootwrap to namespace access

Changes
* adding configuration options for rootwrap
* refactoring ssh connection to use rootwrap as a proxy when requested
* adding documentation for rootwrap configuration
* adding default rootwrap filters file
* adding default sudoers conf file for sahara user
* adding default rootwrap conf file for sahara-rootwrap
* adding sahara-rootwrap cli script
* adding requirement for oslo.rootwrap

Change-Id: I7871400b2342a4cd1a8910ae5121b1bfdc46078d
Closes-Bug: #1271349
Merge "Adding support for oslo.rootwrap to namespace access"