Allow easy gdk.yml key lookups

Using ./bin/config_read_key we can now lookup values within gdk.yml if it exists, falling back on gdk.example.yml for a default. A key can be as simple as port or multiple levels like sshd_enabled.

gdk.yml with port specified

gdk.yml

---
port: 2000

$ ./bin/config_read_key port
2000

$ echo port | ./bin/config_read_key
2000

gdk.yml with no port specified

gdk.yml

---

$ ./bin/config_read_key port
3000

$ echo port | ./bin/config_read_key
3000

gdk.example.yml is consulted for the default value which is 3000 in this case.

gdk.yml with sshd enabled specified

gdk.yml

---
sshd:
  enabled: true

$ ./bin/config_read_key sshd_enabled
true

$ echo sshd_enabled | ./bin/config_read_key
true