Fix broken if statement in lib/tls on ZSH

When using ZSH, the line `if [[ (!$cert && !$key && $ca) ]]` fails
due to a syntax error.  Instead of checking the variables as a boolean,
we can simply check if they have a non-zero length.  This works in ZSH.

Change-Id: I171ed10a8c0af354e82bd6119508a0c44b6bcd9c
Merge "Fix broken if statement in lib/tls on ZSH"
Merge branch 'master' of github.com:openstack-dev/devstack