Unify rule templates parameters units validation on PMM and SAAS sides

Parameter unit in yaml file is just a string, but in API it's enum. We should unify validation rules for yaml and API.

How to test:

1)  Checked part:  (TESTED AND DEPLOYED)

• make sure nothing breaks

• check that API returns set of IA templates in format :

Instead of:

2) pmm-managed part:

• Check that template files put in /srv/ia/templates/ valid only when param unit is empty, `s` or `%`. In other case such template should not be loaded.

• Check that template allows empty range and default value for parameters

• Check that we can create rule form template without defined default values for parameter only when we specify value in rule creation params.

• Check that we can skip param value when it has default value in template.