Set the default ring in SPK

[mtarng]

As a:
spk user
I want:
to be able to set my default ring
So that:
I can point which ring is my "master" ring/branch
Describe the solution you'd like:
a new spk ring set-default <ring-name> command to set the default ring for a spk managed project.

Acceptance Criteria:

• Create a new command spk ring set-default <ring-name>
• Command sets the default the ring to the bedrock.yaml

Does this require updates to documentation?:
Yes

[andrebriggs]

@mtarng thanks for creating this. Would love to see this in a user guide as mentioned in #955 before implementing.