Add alert for Fastly cost

If the cost exceeds $0, it indicates that we have run out of credit and/or are doing something wrong, in either case we want to be alerted.

host_vars/monitoring.archlinux.org/vault_fastly.yml

+$ANSIBLE_VAULT;1.1;AES256
+32326533316233336234643230396235366233656366666434373939376530323066383263656430
+3262616564623638653562383861353837646336313662660a633364333463363230313437303134
+37303165383030383765383331646162646238633131636639376333313932633333666266373361
+3436666263363766390a613362333865353161353333336134663066326463633237346632613265
+36623930316361653838336161323063363633656333313233353234613335383635626261386233
+66363565656136336437623734613737616463373861663263316131303434613734653833646566
+35623064343134386533386237393536313536646539656265646266346232336565393564333232
+33623930393361643935333661646462316463636264653931373032643137616564636438616364
+39316564316331613135623565393766363563653935376336643832366132383764