Add new configallowinsecurerepositories to the test framework

Add new configallowinsecurerepositories to the test framework

Add a new configallowinsecurerepositories that controls the value
of Acquire::AllowInsecureRepositories for the tests. Set it to
"false" for most of the testsuite and only enable it where its
really needed. We want to switch the default for this post-jessie.