Deprecate logdir parameter (use log_dir instead)

The logdir parameter is deprecated since grizzly, we now use log_dir
instead. (the retro-compatibility is kept)
Cosmetics: align all parameters of nova class.

Closes-bug: #1282295
(cherry picked from commit 34a961c4d6afafda6fa37e1a3689de9f17bbe65c)
Change-Id: I20b3dee628205d53779afc4db7506a0701ad1bd4
Allow log_dir to be set to false in order to disable file logging

This commit allows a user to disable logging to a directory (for
example if syslog is enabled) in order not to login twice.

Closes-bug: #1282296
(cherry picked from commit 75ae5e5bed4b77e1dd593e3daae479aa8eee23f4)
Change-Id: I0e64cf62f556d40d9243b05b4d0253dddc42b686
Merge "Deprecate logdir parameter (use log_dir instead)" into stable/havana
Merge "Allow log_dir to be set to false in order to disable file logging" into stable/havana