Refactor libcloud.common.base.Connection to avoid doing duplicate work and
remove some additional code.

Also update affected code.
docs: Update "SSL Certificate Validation" section.
Update a link to point to the correct page.
Remove support for old and deprecated VERIFY_SSL_STRICT variable.

This variable was only used in an old version when we enabled cert validation
to ease the migration path. It has been deprecated since then.
Set LibcloudHTTPSConnection.verify variable to True by default.
Use unittest2 and assertRaisesRegexp instead.
Remove unused import.
Update upgrade notes.