Allow multisite networks to be initialized without adding the config file.
`core install-network`: Don't append multisite constants to the wp-config if they already exist.
Check for the state of wp-config outside the modify_ method.

See https://github.com/wp-cli/wp-cli/pull/486#discussion_r4492066
Better verbosity for `wp core install-network`

See https://github.com/wp-cli/wp-cli/pull/486#issuecomment-18811537
A basic test for converting an install to multisite.
Simpler way to do this
behat: try running the install-network command again
use regex to search through wp-config.php