Bump version number to 1.3.0~alpha1

Reviewed-by: iustinp

configure.ac

 # Configure script for Ganeti
 m4_define([gnt_version_major], [1])
-m4_define([gnt_version_minor], [2])
-m4_define([gnt_version_revision], [3])
-m4_define([gnt_version_suffix], [])
+m4_define([gnt_version_minor], [3])
+m4_define([gnt_version_revision], [0])
+m4_define([gnt_version_suffix], [~alpha1])
 m4_define([gnt_version_full],
           m4_format([%d.%d.%d%s],
                     gnt_version_major, gnt_version_minor,