move caphost creation in a postprovision hook due to idempotency pb

[jfomhover]

Purpose

Capability host creation can happen only once. Running the bicep twice will lead to an error. We need to move caphost creation in a postprovision hook that will test if it already exists and create it only if it doesn't.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

[JeffreyCA]

Consider using the @onlyIfNotExists() decorator instead?