enable simple region definition in config.

Nova uses long format for describing regions in config -
region_list=ROne=IP1,RTwo=IP2

this change enables simple definition without IP(mostly used for devstack instalation)
region_list=RegionOne

Change-Id: I2d2d4d111df6bb21ddc02d4341d8a9a726300e36

ec2api/api/availability_zone.py

@@ -103,6 +103,8 @@ def describe_regions(context, region_name=None, filter=None):
         regions = []
         for region in CONF.region_list:
             name, _sep, host = region.partition('=')
+            if not host:
+                host = CONF.ec2_host
             endpoint = '%s://%s:%s%s' % (CONF.ec2_scheme,
                                          host,
                                          CONF.ec2_port,