Add ability to retain specified fds open in RunCmd

Passing tap devices to KVM as file descriptors requires that the respective
file decriptors remain open during utils.RunCmd execution. To this direction,
we add a “noclose_fds” keyword argument to utils.RunCmd, accepting a list of
file descriptors to keep open. The actual fd handling is implemented in
_RunCmdPipe and _RunCmdFile using subprocess.Popen's “preexec_fn”[1],
since subprocess.Popen provides no other way to selectively handle fds.

A small modification is also made to test/ganeti.utils_unittest.py to comply
with _RunCmdPipe's new API and a new unit test is added to test the selective
fd retention functionality.

[1] “If preexec_fn is set to a callable object, this object will be called in
     the child process just before the child is executed. (Unix only)”
    Subprocess documentation

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>