Add DriverFactory

This patch create a new class called DriverFactory that will be used to
manage the loaded drivers in Ironic. Before the list of loaded drivers
existed only inside the NodeManager class making it difficult to get
any information about the drivers outside that class.

Change-Id: I56a89dab6ed8f323dcc335c9921f7d0edec634ec
Remove the NULL state

NULL and NOSTATE seems to be redundant, plus the NULL state haven't been
used anywhere.

Change-Id: Iaf20dcc661c4334fe2b140306ee2310a83478bb6
Do not use string concatenation for localized strings

Change-Id: I974917c363ba7eb152d2487899b600fa0355e938
Merge "Do not use string concatenation for localized strings"
Merge "Add DriverFactory"