Bump version to 2.1.0~beta0

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

configure.ac

 # Configure script for Ganeti
 m4_define([gnt_version_major], [2])
-m4_define([gnt_version_minor], [0])
-m4_define([gnt_version_revision], [4])
-m4_define([gnt_version_suffix], [])
+m4_define([gnt_version_minor], [1])
+m4_define([gnt_version_revision], [0])
+m4_define([gnt_version_suffix], [~beta0])
 m4_define([gnt_version_full],
           m4_format([%d.%d.%d%s],
                     gnt_version_major, gnt_version_minor,