QB Role `default_dir` Strategies

Details on the acceptable values for the default_dir metadata key for QB roles, which I'm calling strategies.

null (or missing entirely) - No strategy, DIRECTORY arg required on CLI

No strategy for figuring out a default directory.

A QB::UserInputError will be raised by QB::Role#default_dir
false - DIRECTORY arg not used

Role does not use a directory argument.

Providing one via the command line should result in an error (though this is untested), and qb_dir will not be set in Ansible.
Strategies with no parameters

Encoded as simple string values.
- 'cwd' - Current working directory
  
  Use the working directory the run command was executed in.
  
  This is expected to pretty much always succeed.
  Example
```
default_dir: cwd
```
- 'git_root' - Git repo root
  
  Use the root of the Git repo for the working directory the run command was executed in.
  
  Raises an error if that directory is not part of a Git repo.
  Example
```
default_dir: git_root
```
Strategies with parameters

Encoded as directories with a single string key denoting the strategy pointing to a value holding the parameters.
- {exe: <path:string>} - Run an executable
  
  Run the executable at path and use the output, which should be a single string directory path (we do chomp what we get back).
  
  The exe is fed a JSON dump of the role options at the time via STDIN, so it can dynamically determine the directory based on those if it wants.
  
  If path is relative, it's assumed to be relative to the role directory, NOT the working directory.
  
  Raises errors when things go wrong.
  Example Role-relative executable
  
  This will attempt to resolve the default directory by executing the <role_path>/bin/get_default_dir file
```
default_dir:
  exe: bin/get_default_dir
```
- {find_up: <rel_path:string>} - Find a path walking up
  
  Walk up the directory hierarchy from the CLI working directory looking for the first place that rel_path exists.
  
  This is used to do things like find the Gemfile, nearest .gitignore, etc:
  Example Find the nearest Gemfile walking up directories
```
default_dir:
  find_up: Gemfile
```
  Example Find the nearest .gitignore walking up directories
```
default_dir:
  find_up: .gitignore
```
  rel_path can be deep too:
  Example Find the nearest directory walking up for which <dir>/dev/ref exists
```
default_dir:
  find_up: dev/ref
```
- {from_role: <role:string} - Use value from another role
  
  Use the value from another QB role, which this one presumably includes.
  
  role can be anything you would normally use to locate a role using qb run, etc. (partial or full name or path).
  Example Use the value for the qb/ruby/bundler role
```
default_dir:
  from_role: qb/ruby/bundler
```
Array - Multiple strategies (fallbacks)

A list of strategies to try; first to succeed wins.
Example Closest Gemfile or Git root

Walk up directories looking for Gemfile, and if one can't be found then use the Git repo root.
```
default_dir:
-   find_up: Gemfile
-   git_root
```

QB Role default_dir Strategies

QB Role `default_dir` Strategies