Add multi default orgs

[sergeohl]

With this PR I add a way to subscribe in several organizations by default.

Like that:

AUTH_LDAP_DEFAULT_SENTRY_ORGANIZATION = u'My Organization Name,Sentry'

[sergeohl]

any news ?

[Bessonov]

I'm not sure, that it's sofisticated. From my point of view the mapping should happens on LDAP group level, like:

AUTH_LDAP_DEFAULT_SENTRY_ORGANIZATION = {
  ldap_group_x: [u'Sentry organization x'],
}

[azmeuk]

@sergeohl any chance to see improvements on this PR soon?