Reduce instance sizes in gce

tests/files/gce_centos7-flannel-addons.yml

 # Instance settings
 cloud_image_family: centos-7
 cloud_region: us-central1-c
-cloud_machine_type: "n1-standard-2"
+cloud_machine_type: "n1-standard-1"
 mode: ha
 
 # Deployment settings

tests/files/gce_coreos-calico-aio.yml

 # Instance settings
 cloud_image_family: coreos-stable
 cloud_region: us-central1-a
-cloud_machine_type: "n1-standard-2"
+cloud_machine_type: "n1-standard-1"
 mode: aio
 ##user-data to simply turn off coreos upgrades
 startup_script: 'systemctl disable locksmithd && systemctl stop locksmithd'

tests/files/gce_coreos-vault-upgrade.yml

 # Instance settings
-cloud_machine_type: "n1-standard-4"
+cloud_machine_type: "n1-standard-1"
 cloud_image_family: coreos-stable
 cloud_region: us-central1-b
 mode: aio

tests/files/gce_ubuntu18-flannel-aio.yml

 # Instance settings
 cloud_image_family: ubuntu-1804-lts
 cloud_region: us-central1-a
-cloud_machine_type: "n1-standard-2"
+cloud_machine_type: "n1-standard-1"
 mode: aio
 
 # Deployment settings