Add a deploy lock for commands that are unsafe to run concurrently.
The lock is taken by creating a `mrsk_lock` directory on the primary
host. Details of who took the lock are added to a details file in that
directory.
Additional CLI commands have been added to manual release and acquire
the lock and to check its status.
```
Commands:
mrsk lock acquire -m, --message=MESSAGE # Acquire the deploy lock
mrsk lock help [COMMAND] # Describe subcommands or one specific subcommand
mrsk lock release # Release the deploy lock
mrsk lock status # Report lock status
Options:
-v, [--verbose], [--no-verbose] # Detailed logging
-q, [--quiet], [--no-quiet] # Minimal logging
[--version=VERSION] # Run commands against a specific app version
-p, [--primary], [--no-primary] # Run commands only on primary host instead of all
-h, [--hosts=HOSTS] # Run commands on these hosts instead of all (separate by comma)
-r, [--roles=ROLES] # Run commands on these roles instead of all (separate by comma)
-c, [--config-file=CONFIG_FILE] # Path to config file
# Default: config/deploy.yml
-d, [--destination=DESTINATION] # Specify destination to be used for config file (staging -> deploy.staging.yml)
-B, [--skip-broadcast], [--no-skip-broadcast] # Skip audit broadcasts
```
If we add support for running multiple deployments on a single server
we'll need to extend the locking to lock per deployment.
`docker image ls` doesn't tell us what the latest deployed image is (e.g
if we've rolled back). Pull the latest image tag through to the server
so we can use it instead.
