Bugfix instance create when file-storage-dir None
Bugfix instance create when file-storage-dir None

os.path.join does not like None as argument and fails with
AttributeError: 'NoneType' object has no attribute 'startswith'.

This patch makes sure the passed argument is a string in any case.

Reviewed-by: iustinp


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