[Openstack-i18n] Some hacks on gettext for separate pot file generation

Hi, Recently my colleagues pointed out that gtk+ project already support multiple pot files, it supports both _("msg") and P_("msg") notation. In gtk+ project, it has two directories, po for _("msg"), po-properties for P_("msg"). The po directory is the same as other projects, the po-properties directory has modified the po-properties/Makefile.in.in file, to recognize P_("msg"). It uses "XGETTEXT_KEYWORDS = --keyword --keyword=P_" to extract the P_("msg") messages. Actually this approach requires one directory for each pot file, maybe good to not introduce many gettext directories for openstack projects. Thanks, Peng Wu