<install> Set default BIND key algorithm to MD5

Change the default settings for CONF_BIND_KEYALGORITHM and
CONF_BIND_KEYSIZE to HMAC-MD5 (instead of HMAC-SHA256) and 512 (instead of
256), respectively, so that the DNSSEC key generation, BIND configuration,
and dns-nsupdate plug-in configuration will use HMAC-MD5 (instead of
HMAC-SHA256) by default.

OpenShift Enterprise 2.0 only supports the HMAC-MD5 algorithm for nsupdate.

This commit fixes bug 1066171.