add plugin to gradle buildscript from a local jar

You should create a directory with your *.jar libs and add flatDir {} in repositories {} block to initialize the directory that keeps jar-libs as a repository:

buildscript {
    repositories {
        jcenter()
        flatDir { dirs 'jarlibs' } // *.jar libs are keep in 'app/jarlibs' dir
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.2'
        classpath 'com.google.gms:google-services:3.0.0' // now it works
    }
}

I was able to add the plugin from the jar by adding the jar in my lib folder and call it from the project gradle dependencies like so :

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.2'
        classpath fileTree(include: ['*.jar'], dir: 'app/libs')
        classpath files('app/libs/google-services-3.0.0.jar')

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}