[Buildbot] #3511: LdapUserInfo: make group features optional

[Buildbot trac]

#3511: LdapUserInfo: make group features optional
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Reporter:  gracinet     |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  major        |  Milestone:  undecided
 Version:  0.8.12       |   Keywords:
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 There are some LDAP directories that don't have groups, but
 `groupMemberPattern` etc are mandatory in `LDAPUserInfo`.

 It would be nice to make them optional. In that case the returned groups
 would always be an empty list.

 I can implement that easily (better to do it before 0.9 because that's a
 slight break in API, due to the arguments being positional in the
 constructor)

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Ticket URL: <http://trac.buildbot.net/ticket/3511>
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