Adds vnc_keymap support

The nova.conf file handle a configuration option called vnc_keymap.
This option let operators define which keymap will be used in VNC
sessions.

This commit will let puppet handle this option in nova.conf.

vnc_keymap is an optional parameters and its default value is
'en-us'.

Change-Id: I7ba49a0ef1e48adbf757c7515a67960d0007a8f5
Closes-Bug: 1325585