Inject global job parameters

[haklop]

Enable job parameters to be set on project side.

Can be use to defined default parameters values.

Add a new command: bzk project env

Usage: bzk project env  COMMAND [arg...]

View or modify a bazooka project environment variable

Commands:
  list         List project environment variables
  get          Get a specific project environment variable
  set          Set a specific project environment variable
  unset        Delete a specific project configuration key

bzk project env my-project A 5 sets the default value of A to 5 for the project my-project. This value is used if a job is started without specify a value for A.

Permutation is not yet possible (wip, it'll come on another PR). Permutations can only be generated by using bzk job start command.

[julienvey]

Tested, LGTM

[jawher]

s/SetEnv/Set/

[jawher]

Just so we don't forget, we agreed with @haklop that project env should be multivalued, i.g:

bzk project env set GOMAXPROCS 1 2 3 4

[jawher]

And don't forget to update the doc to explain the 3 override levels: bazooka.yml -> project env -> job parameters.