Person.addMember() does not properly invalidate the person's _inTeam_cache

The following test suite code doesn't work as expected:

    >>> team_two = personset.newTeam(ddaa, 'team-two', 'Team Two')
    >>> from canonical.lp.dbschema import TeamMembershipStatus
    >>> sabdfl = personset.getByName('sabdfl')
    >>> team_two.addMember(salgado, sabdfl, TeamMembershipStatus.ADMIN)
    >>> flush_database_updates()
    >>> [team.name for team in salgado.getAdministratedTeams()]
    [u'admins', u'rosetta-admins']

Note that salgado is not an administrator for team_two!

The problem is that Person has a _inTeam_cache that doesn't get invalidated by
Person.addMember(). The alternative is to do this:

    >>> from canonical.lp.dbschema import TeamMembershipStatus
    >>> salgado.join(team_two)
    >>> sabdfl = personset.getByName('sabdfl')
    >>> team_two.setMembershipData(salgado, TeamMembershipStatus.ADMIN, sabdfl)
    >>> flush_database_updates()

But this requires that team_two have an OPEN subscription policy. (Extra
dance steps needed if not.)

Person.addMember() should invalidate the _inTeam_cache for the person being
added. Note that .join() is implemented in terms of .addMember() so the
proper fix is probably to move the line:

        self._inTeam_cache = {} # Flush the cache used by the inTeam method

to .addMember(). I found this too late in the 1.1.7 cycle to feel comfortable
about sneaking the fix in.