Enable to select specific network service type

Network service type is selected as 'network' statically.
This will select the service_type from environment variables or CLI options.
The option works like the service type option in the novaclient.

Implements blueprint specify-service-type
Change-Id: If21bd2b5cb35da5bcabca79aa0567870331a3a6f
Merge "Enable to select specific network service type"