Make "multi_host" True when it is set to 'T' in network_create.

Fixes bug 1161297
Even if creating a network with the "--multi_host=T", the "multi_host"
property of the new network is still "False".
This is because nova server interprets 'T' to "False".
This patch fixes the problem.

Change-Id: I171c7dc72cb515c47ea106558080eafa10dee873