Fix CLI support for DVR, take 2

Previously --distributed was displayed as positional
argument in the help text, but acted as an optional
argument with the -- semantics. It now behaves like
any other optional argument.

A side-effect is that previously you could:
1) neutron router-create --distributed r1
2) neutron router-create --distributed=True r1
3) neutron router-create --distributed=False r1

While now only options 2 and 3 are viable.

DocImpact
Partially-implements: blueprint neutron-ovs-dvr
Closes-Bug: #1346121
Change-Id: I60675b4f60fe8154af4ab6d8f0e93867cf362a59