samples: Replace swtpm_localca with script calling swtpm-localca (issue #482)

samples: Replace swtpm_localca with script calling swtpm-localca (issue #482)

Replace the binary swtpm_localca with a script that now calls the binary
swtpm-localca, which is to be installed in $bindir.

So that we can use this for script for calling swtpm-localca during testing
(when it is not installed), write the script in such a way that it de-
termines whether there's swtpm-localca in the same directory and if not call
it from ${bindir}/swtpm-localca.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>