Removes 'positive' tag from tests.

Also removes brackets where tag is a single value. This should be
finally cleanup the tags, see the following:

$ grep -r "@attr" tempest | cut -d: -f2 | sort -u
    @attr(type='gate')
    @attr(type=['negative', 'gate'])
    @attr(type='smoke')
$

Change-Id: I243800c597996c0dbcf153d66946062df6d298ab
Implements: blueprint set-gate-attribute
Merge "Removes 'positive' tag from tests."