Add checks for missing disk space
Add checks for missing disk space

This small patch adds disk space checks to the Cluster.checkData
function, and simplifies a little the warning messages.
andir/ganeti,ganeti/ganeti,grnet/snf-ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,ganeti/ganeti,apyrgio/snf-ganeti,apyrgio/snf-ganeti,yiannist/ganeti,ganeti/ganeti,mbakke/ganeti,apyrgio/ganeti,dimara/ganeti,mbakke/ganeti,leshchevds/ganeti,leshchevds/ganeti,yiannist/ganeti,onponomarev/ganeti,yiannist/ganeti,dimara/ganeti,bitemyapp/ganeti,mbakke/ganeti,bitemyapp/ganeti,apyrgio/ganeti,leshchevds/ganeti,grnet/snf-ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,andir/ganeti