[linter] Linter help_rules.py should lint help objects and not help_entries.

[linter] Linter help_rules.py should lint help objects and not help_entries. We will soon [support](https://github.com/Azure/azure-cli/projects/8) help.yaml alongside help.py. Linter rules should be independent of help_entry type.

Secondly, linter should handle parsing incorrect datetime's or condition values. ie. monitor --start-time and --condition parameter. Should raise appropriate error.

Linter should also validate parameter values to the extent that this is possible / reasonable. E.g. enum values, true false, etc.