if keystore is given as arg to init, create keystore if it does not exist

if keystore is given as arg to init, create keystore if it does not exist

Previously, `fdroid init --keystore /tmp/foo` expected the keystore to
exist, or it quit with an error.  But I've changed my mind, I think it is
useful to have it generate a new keystore at that location if it does not
exist.  For example, in tests/run-tests.sh. It still will not clobber an
existing file at that location.