DPE-1979 Avoid assumption that hostnames of members in cluster are resolvable

[shayancanonical]

Issue

1. We are assuming that hostnames of cluster members are resolvable from every cluster node (which holds true in lxc). However, it is an incorrect assumption for nodes in openstack
2. We are setting the unit in BlockedStatus in the update-status event handler when there really isnt anything a user would need to do. The error is potentially recoverable since the update-status event handler will fire again (periodically)

Solution

1. Set report_host for every member of the cluster. This will automatically set group_replication_local_address based off the value of report_host.
2. Change the status to MaintenanceStatus

[codecov]

Codecov Report

Merging #214 (47dcc17) into main (2a9839e) will decrease coverage by 0.30%.
The diff coverage is 21.42%.

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
- Coverage   63.83%   63.54%   -0.30%     
==========================================
  Files          12       12              
  Lines        2201     2219      +18     
  Branches      278      283       +5     
==========================================
+ Hits         1405     1410       +5     
- Misses        711      722      +11     
- Partials       85       87       +2     

Impacted Files	Coverage Δ
src/charm.py	46.36% <18.51%> (-0.55%)	⬇️
lib/charms/mysql/v0/mysql.py	74.28% <21.42%> (-0.72%)	⬇️
src/mysql_vm_helpers.py	59.01% <100.00%> (+0.54%)	⬆️

[paulomach]

lgtm