Auto version-check

In 2.1 we have --version-check (default: off). Possible values are off, auto, http, https.

In 2.2 we're going to have --[no]version-check (default: on). If IO::Socket::SSL is installed, https will be used; else, http will be used.

One problem this creates for development is: we do _not_ want --version-check on during testing, else we'll flood the v-c database with reports from all our various test machines. One way to globally disable this feature is by adding "no-version-check" to /etc/percona-toolkit/percona-toolkit.conf. sandbox/test-env now requires this.