bootstrap tab: "active" class is added several times, and to several tabs

When used as class attributes on a form, the bootstrap.Container and bootstrap.TabHolder helpers are not properly reinitialized, which leads to duplicate css "active" attribute, and (in case of error on tab) to several tabs being active together.

Example:
```
class TestForm(forms.Form):
    val1 = forms.CharField(required=False)
    val2 = forms.CharField(required=True)
    helper = FormHelper()
    helper.layout = Layout(
        TabHolder(Tab('one', 'val1',),
                        Tab('two', 'val2',)))

test_form = TestForm()
html = render_crispy_form(test_form)
... 
<div id="one" class="tab-pane active"
...
# render again
test_form = TestForm()
html = render_crispy_form(test_form)
...
<div id="one" class="tab-pane active active"
...

# ...and again
test_form = TestForm()
html = render_crispy_form(test_form)
...
<div id="one" class="tab-pane active active active"
...

# render with incomplete POST (form error)
test_form = TestForm(data={'val1':'foo'})
html = render_crispy_form(test_form)
# both tabs are active now
...
<div id="one" class="tab-pane active
...
<div id="two" class="tab-pane active
...
```

Attached patch and test fixes this for Tab/Container