fix: [DPE-7404] quorum loss recovery and test fixes

[paulomach]

Issue

• Test for quorum loss is not really provoking a quorum loss.
• Corrected test expose other issues with the implementation of the promote action and rejoin procedures.
• charm automated GR instance recovery goes boom when units try to rejoin at the same time.

Solution

• Fix test to ensure two (out of three) instances are killed at the same time (instead of one after the other), which will cause quorum loss.
• Fix call to shell api command that recovers from quorum loss.
• Serialize the recovery of the GR instances to avoid GR hang.
• Tweaked status setting to better display quorum loss.