Fixes #14730 - Only search host proxies explicitly

When searching hosts with free text in a taxonomy scope, if the taxonomy
has "all smart proxies" associated, it leads to scoped search generating
an incorrect scope which causes an invalid query:
`SELECT smart_proxies.id FROM "smart_proxies" "puppet_ca_proxies_hosts"`
Making the puppet_ca search only explicitly prevents this at the cost of
not allowing to search by puppet_ca using free text. I believe this is a
reasonable tradeoff as I did not find a simpler way to handle this (might
require some deep tweeking in scoped search).
To maintain consistency I also made the puppetmaster search explicit
only.