Makes designated HOV way's routability configurable in profiles.

Why only `hov=designated` and not all access tags, such as `hov:yes`,
`hov=no` and so on? From the Wiki:

- designated: The way is designated to high occupancy vehicles.
- yes: High occupancy vehicles are allowed. This by itself does not imply that other vehicles are restricted from using the way.
- no: High occupancy vehicles are not allowed on the way. This by itself does not imply that other vehicle types are allowed to use it.

The primary use-case is conditionally filtering ways such as:

http://www.openstreetmap.org/way/11198593#map=19/37.82571/-122.30521&layers=D

In addition there is a notion of HOV lanes for lane handling:

http://wiki.openstreetmap.org/wiki/Key:hov#hov:lanes.3D.2A

This changeset does not handle lanes at all, only designated HOV ways.
For HOV lane support, a logic similar to the lane access handling needs
to be implemented. This needs to go hand in hand with the existing lane
handling introduced in:

https://github.com/Project-OSRM/osrm-backend/commit/7d076e93445a4d016422a575f1132b267cd13c29

References:
- #2711
- http://wiki.openstreetmap.org/wiki/Key:access
- http://wiki.openstreetmap.org/wiki/Key:hov#Values

features/car/access.feature

@@ -5,7 +5,7 @@ Feature: Car - Restricted access
     Background:
         Given the profile "car"
 
-    Scenario: Car - Access tag hierarchy    on ways
+    Scenario: Car - Access tag hierarchy on ways
         Then routability should be
             | access | vehicle | motor_vehicle | motorcar | bothw |
             |        |         |               |          | x     |
@@ -148,3 +148,10 @@ Feature: Car - Restricted access
             | primary |      |         | no  |           | x     |
             | runway  |      |         |     | yes       |       |
             | primary |      |         |     | no        | x     |
+
+    Scenario: Car - only designated HOV ways are ignored by default
+        Then routability should be
+            | highway | hov        | bothw |
+            | primary | designated |       |
+            | primary | yes        | x     |
+            | primary | no         | x     |