[JUJU-3584] Enable updating/setting owner labels by using secret-get

[ycliuhw]

This PR implements updating/setting owner labels by using secret-get for:

• owner units;
• leader unit for application owned secrets;

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
• Integration tests, with comments saying what you're testing
• doc.go added or updated in changed packages

QA steps

juju exec --unit snappass-test/0 'secret-add ownedby=snappass-test'
secret:ch52creffbas7aip1bjg

juju exec --unit snappass-test/0 'secret-add ownedby=snappass-test/0 --owner unit'
secret:ch52csmffbas7aip1bk0

juju exec --unit snappass-test/0 -- secret-get ch52creffbas7aip1bjg --label=snappass-test
ownedby: snappass-test

juju exec --unit snappass-test/0 -- secret-get --label=snappass-test
ownedby: snappass-test

juju exec --unit snappass-test/1 -- secret-get --label=snappass-test
ownedby: snappass-test

juju exec --unit snappass-test/0 -- secret-get ch52csmffbas7aip1bk0 --label=snappass-test/0
ownedby: snappass-test/0

juju exec --unit snappass-test/0 -- secret-get --label=snappass-test/0
ownedby: snappass-test/0

Documentation changes

No

Bug reference

https://bugs.launchpad.net/juju/+bug/2017042

[ycliuhw]

/merge