don't break get_ip_address if static IP is assigned

[johnswanson]

If the first network in the config specifies a static IP address,
then the key to look up in addresses is network[:name], not the
bare network hash.

[coveralls]

Coverage decreased (-0.04%) to 92.188% when pulling 99896ed on johnswanson:master into 7a5d972 on ggiamarchi:master.

[ggiamarchi]

@johnswanson testing your code, i realize there's still non covered situations. As you pointed out, in the Vagrantfile a network can be either a string or a hash. But that's not all. When it's a string, it can be either the network name or the network id. And when it's a hash, it can contains either a key name or a key id.

If i summarize,

First network format	before your PR	your PR
String name / "private-net"	✅	✅
String ID / "adbcd056-2a60-4404-aa2b-c31bd9d2841a"	❌	❌
Hash with name / { name: "private-net" }	❌	✅
Hash with ID / { id: "adbcd056-2a60-4404-aa2b-c31bd9d2841a" }	❌	❌

If you meant to fix the whole issue it would be awesome, but as long as your patch partially fix it i don't have any objection for merging it and create an issue for the rest.

Let me know your preference.