Bump version to 2.0.0~alpha0

We decided to bump the major number to 2 a few weeks ago due to the huge number
of changes going into it.

Reviewed-by: iustinp

configure.ac

 # Configure script for Ganeti
-m4_define([gnt_version_major], [1])
-m4_define([gnt_version_minor], [3])
+m4_define([gnt_version_major], [2])
+m4_define([gnt_version_minor], [0])
 m4_define([gnt_version_revision], [0])
-m4_define([gnt_version_suffix], [~alpha1])
+m4_define([gnt_version_suffix], [~alpha0])
 m4_define([gnt_version_full],
           m4_format([%d.%d.%d%s],
                     gnt_version_major, gnt_version_minor,