ima: initialize the "template" field with the default template

IMA policy rules are walked sequentially.  Depending on the ordering of
the policy rules, the "template" field might be defined in one rule, but
will be replaced by subsequent, applicable rules, even if the rule does
not explicitly define the "template" field.

This patch initializes the "template" once and only replaces the
"template", when explicitly defined.

Fixes: 19453ce0 ("IMA: support for per policy rule template formats")
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>