This PR disables the automatic retrying of Utils/http_request() that was skipping certificate validation.

The new default is to produce a hard error on certificate errors.

This PR also adds a new --insecure flag to the cli update command that turns this retrying back on when explicitly requested, which allows manually going back to the previous behavior.

Subsequent PRs on the command repositories will add a similar --insecure flag to the appropriate commands on the following repositories:

• wp-cli/config-command => Add --insecure flag to config create & config shuffle-salts commands config-command#128
• wp-cli/core-command => Add --insecure flag to core download & core update commands core-command#186
• wp-cli/extension-command => Add --insecure flag to plugin|theme install & plugin\theme update commands extension-command#287
• wp-cli/checksum-command => Add --insecure flag to core|plugin verify-checksums commands checksum-command#86
• wp-cli/package-command => Add --insecure flag to package install command package-command#138