Contributor agreements and licenses are both exactly what they say on the tin. Thus, you cannot license a project under a contributor agreement, and you cannot conduct relations with your contributors under a license.
A license defines the terms under which people are allowed to use your product. A contributor agreement defines the terms under which people are allowed to contribute to your product. Of course, as the owner you can accept or reject any changes you like, but the CA defines the accepted baseline by which all contributors are treated, so nobody complains of mistreatment.