render retry configuration in template, remove retry_interval attribute

[Thanhphan1147]

The retry_interval attribute is removed since there's no way to configure that in HAproxy. To configure the time between 2 retries we can use timeout.connect.

Fixes #160

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, complex)

[Thanhphan1147]

Integration tests will be added shortly

[github-actions]

Test results for commit 115bea0

Test coverage for 115bea0

Name                                     Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------------------------
lib/charms/haproxy/v1/haproxy_route.py     343     57     80     25    79%   171, 174, 222, 231-234, 259-262, 283-288, 572-573, 716-719, 723, 749-760, 783-786, 790-792, 811-813, 822, 974-980, 984, 1157->1159, 1161->1163, 1163->1165, 1165->1167, 1167->1169, 1169->1172, 1205, 1210, 1213, 1238, 1266, 1270, 1274, 1297, 1317, 1326-1327, 1329->exit, 1365-1367, 1387, 1401, 1406-1408
src/charm.py                               221     79     64      7    56%   82, 178, 186-199, 204, 209, 225, 236, 242-243, 271-291, 310-326, 386-399, 412-417, 426, 438-452, 457, 467, 473-479, 489-495
src/haproxy.py                              94     37      2      0    59%   95, 104-110, 128-142, 155-164, 172-179, 188-197, 206-212, 220-229, 257, 281-283
src/http_interface.py                       73     25      4      0    62%   74, 83, 92, 106-108, 126, 138, 150, 162, 170-175, 187, 194, 202, 217-227
src/state/charm_state.py                    72     15     14      4    78%   89-91, 96-97, 100, 137-142, 151, 193-195, 207-208
src/state/exception.py                       1      0      0      0   100%
src/state/ha.py                             30      1      2      1    94%   50
src/state/haproxy_route.py                 105     19     28      5    76%   100, 114-117, 131-135, 149-157, 211, 221-222, 253, 261
src/state/ingress.py                        35      0      4      0   100%
src/state/ingress_per_unit.py               31      0      4      0   100%
src/state/tls.py                            38      5     10      3    83%   76, 114-119, 125-126
src/state/validation.py                     46     23      8      1    44%   66-67, 71-98
src/tls_relation.py                         43      5     12      4    84%   73-74, 76->75, 99-105, 117->119
------------------------------------------------------------------------------------
TOTAL                                     1132    266    232     50    72%

Static code analysis report

Run started:2025-08-04 12:57:13.910694

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 4256
  Total lines skipped (#nosec): 4
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 3

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):