fix rocks add host on rocks 5.6 at install time

rocks add host compute-0-2 os=linux
does not work at installation time

This is becuase MySQL-python of centos 5.6 sql sets returns a set of Sets
(Set(['linux']),) so we need to pop out the first containing set
on rocks 6 it is just ('linux',) so no need to do this

Why this problem re-appears only at install time on rocks 5.6 it is
one of the new secret of Fatima...

I suspect another interaction between anaconda python and
foundation-python but I have not been able to pinpoint it

Signed-off-by: Luca Clementi <luca.clementi@gmail.com>