Add a global lock file to Certbot (#4369)

* add fasteners as a dependency

* add LOCK_FILE constant

* Add lock file to Certbot

* Move code to _run_subcommand

* move lock file path into CLI_CONSTANTS

* add --lock-path flag

* move locking code to separate function

* Add TestAcquireFileLock

* assert we log

* test lock contention

* add fasteners to certbot-auto

* Use a different lock file for each test in MainTest

certbot/constants.py

@@ -33,6 +33,7 @@ CLI_DEFAULTS = dict(
     auth_cert_path="./cert.pem",
     auth_chain_path="./chain.pem",
     strict_permissions=False,
+    lock_path="/tmp/.certbot.lock",
     debug_challenges=False,
 )
 STAGING_URI = "https://acme-staging.api.letsencrypt.org/directory"