Add a new command line option `--skip-tag` that will skip tagging
after a successful build. The help text is copied from the same
option of other sub-commands in the koji client. The hub plugin's
jsonschema was updated accordingly, and the builder plugin will
skip the tag if the option was requested.
Tests were added or augmented for all three plugins to test the
new option.
Create a new plugintest.PluginTest class that shares the plugin
loading code that is common to all plugin testing. Adapt the
existing (hub, builder) tests.
Also correct the name for test_builder.TestHubPlugin to
TestBuilderPlugin.
