LXC: Refactoring code creating blank file
LXC: Refactoring code creating blank file

Add the function _CreateBlankFile which can be used to create a blank
file.
Replace existing code that uses utils.WriteFile to use this function.

Signed-off-by: Yuto KAWAMURA(kawamuray) <12589a13a27e6c6eeb0b2b8963a6f3e92691a9e2@gmail.com>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

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