vstart: move common rgw config to [client.rgw]

vstart: move common rgw config to [client.rgw]

multisite tests in src/test/rgw/test_multi.py run vstart.sh with RGW=0
because it does some extra configuration before starting radosgw with
src/mrgw.sh. the result is that vstart.sh doesn't add extra config
sections for [client.rgw.{port}], so these settings don't take effect

moving the common config into [client.rgw] allows the settings to apply
without relying on vstart.sh to create the additional sections for each
gateway

Signed-off-by: Casey Bodley <cbodley@redhat.com>