The simplest and cleanest way to add an external configuration is through a separate Gradle file
build.gradle
apply from: './keystore.gradle'
android{
    signingConfigs {
        release {
            storeFile file(keystore.storeFile)
            storePassword keystore.storePassword
            keyAlias keystore.keyAlias
            keyPassword keystore.keyPassword
        }
    }
}
keystore.gradle
ext.keystore = [
    storeFile    : "/path/to/your/file",
    storePassword: 'password of the store',
    keyAlias     : 'alias_of_the_key',
    keyPassword  : 'password_of_the_key'
]
The keystore.gradle file can exist anywhere in your file system, you can specify its location inside the apply from: '' at the top of your gradle file or at the end of your main project build.gradle file.
Typically its a good idea to ignore this file from version control system such as git if its located inside your repo.
It is also a good idea to provide a sample keystore.gradle.sample which developers entering the project would rename and populate on their development machine. This file would always be contained inside the repo at the correct location.