Adjust TC election target to 6 weeks before summit

The existing 3 weeks prior to summit target for TC elections can be
problematic for travel planning for candidates who might only go to
summit if they win their election, or might plan a different length
of trip depending on their role(s) in the community. This change
makes the target for the election to be six weeks prior to summit
to ease that planning.

In addition to helping with travel concerns, it also means that any
newly elected TC members will be more involved in planning for the
forum at the summit.

If approved this change would go in effect for the second election
in 2018. The first election of 2018 is already scheduled.

Change-Id: Ie1a7d9bae28864fb7413d8df2daa67abc5c70778

reference/charter.rst

@@ -124,7 +124,7 @@ The 13 TC seats are partially renewed every 6 months using staggered elections:
 6 seats are renewed every (Northern hemisphere) Fall, and 7 seats are renewed
 every Spring. Seats are valid for one-year terms. For this election we'll use a
 multiple-winner election system (see below). The election is held no later than
-3 weeks prior to each OpenStack Summit (on or before 'S-3' week), with
+6 weeks prior to each OpenStack Summit (on or before 'S-6' week), with
 elections held open for no less than four business days.
 
 If a seat on the TC is vacated before the end of the term for which