Add --mac-prefix to gnt-cluster modify manpage
Add --mac-prefix to gnt-cluster modify manpage

Update the manpage for mac-prefix modification in 'gnt-cluster modify'
command.

Signed-off-by: Dimitris Bliablias <56dc08131759a716534a4a0da56f0029eaf4af5d@gmail.com>
Reviewed-by: Jose A. Lopes <0fb9c4f8511df4ec2ae28af879e9d462f6218a2c@google.com>

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