ovinstanceadd should not use GT.M installations that lack libopenvista.so

When ovinstanceadd is used to create a new OpenVista instance, it searches /opt/lsb-gtm for GT.M installations and picks the latest one to create a "gtm" symlink to in the new instance's root directory. The check is rudimentary and assumes that any subdirectories of /opt/lsb-gtm are valid GT.M instances. ovinstanceadd should do a better job of validating candidate directories - they should have the major GT.M binaries present, as well as the openvista-libs binaries libopenvista.so and gtmsignal.