Change temporary disk from readonly to read-write
Change temporary disk from readonly to read-write

Making the temporary disk readonly has absolutely no advantages and it
has the disadvantage that it prevents the OS install helper from
saving temporary data to it, for example, when downloading the OS
install package which is necessary to run the OS scripts.

Signed-off-by: Jose A. Lopes <0fb9c4f8511df4ec2ae28af879e9d462f6218a2c@google.com>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

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