fixes #2124 - only include classes from the host current environment.

This patch introduce a new setting, which decide how would foreman
address a mismatch between host and hostgruop environments.

for example, when puppet classes (directly or indirectly via hostgroups) that do not
belongs to the host current environment, they would silently removed from
the class list that foreman suggests as an ENC.

NOTE: this only effects the new ENC output (/w param classes).