Moved configure_tempest into main
Move main and argument parsing to the top

No changes were made inside the functions.
Refactor add_admin_user

Changed it to `add_admin_to_tenant` since the original name was misleading.
Refactored user creation
Moved user creation out of ClientManager
Refactored flavor creation
Moved flavor creation out of ClientManager
Added docstrings
Refactored image creation
Moved image creation out of ClientManager
Moved do_networks out of ClientManager

The internal workings of do_networks was not changed.
Refactored ClientManager
Removed unnecessary _client suffixes