Move code from ExpandNames to CheckPrereq
Move code from ExpandNames to CheckPrereq

This is needed since only in CheckPrereq we have the nodes locked, and
future import enhancements will need to have access to the export info
during the parameter build.

Signed-off-by: Iustin Pop <acf83a5274f31a43af6b914614365999469c1a31@google.com>
Reviewed-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>

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