set the username that is passed to rpc based on options

set the username for rpc call based on --username passed
in command arguments, also added a unit test

Signed-off-by: Julius Gawlas <julius.gawlas@hp.com>
Acked-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
contrib/site_rpc_client_lib: provide authorization_headers that allow authentication

In my environment, I am authenticating users against an LDAP server. The CLI
does not currently allow users to authenticate, though, so provide an example
site_rpc_client_lib that does.

Changes from v1:
     * Moved authorship info from __author__ attribute to docstring.

Signed-off-by: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>