Add function to determine current configuration version
Add function to determine current configuration version

The 'gnt-cluster upgrade --resume' command needs to determine whether the
configuration has already been updated or not, in order to decide at which
phase to resume. So provide a function to determine the version of the
configuration on file.

Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Thomas Thrainer <98bdeac947eb8ca18146e9ce7f092788ec242cd1@google.com>

apyrgio/ganeti,ganeti/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/ganeti,mbakke/ganeti,grnet/snf-ganeti,andir/ganeti,andir/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,bitemyapp/ganeti,onponomarev/ganeti,mbakke/ganeti,dimara/ganeti,mbakke/ganeti,ganeti/ganeti,yiannist/ganeti,leshchevds/ganeti,ganeti/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,onponomarev/ganeti,dimara/ganeti,grnet/snf-ganeti,leshchevds/ganeti,yiannist/ganeti,yiannist/ganeti