cluster verify: check drbd helper only when drbd enabled
cluster verify: check drbd helper only when drbd enabled

This change makes sure that 'gnt-cluster verify' only
checks for the DRBD usermode helper, if DRBD is actually
enabled.

Signed-off-by: Helga Velroyen <2d0126b1ec6f9ec5cbec4a7012aa14faeaef10f5@google.com>
Reviewed-by: Thomas Thrainer <98bdeac947eb8ca18146e9ce7f092788ec242cd1@google.com>

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