[6.15.z] Fix parametrization in test_positive_reboot_all_pxe_hosts

[Satellite-QE]

Cherrypick of PR: #13949

Problem Statement

Missing indirect=True in parametrization for provision_multiple_hosts fixture which causes,

tests/foreman/api/test_discoveredhost.py:427: in test_positive_reboot_all_pxe_hosts
    for host in provision_multiple_hosts:
E   TypeError: 'int' object is not iterable

Solution

using getattr to set count as default for this fixture, and removing @pytest.mark.parametrize for this fixture, along with mark for pxe_loader fixture which also has bios set as default.

[Satellite-QE]

trigger: test-robottelo
pytest: tests/foreman/api/test_discoveredhost.py -k test_positive_reboot_all_pxe_hosts
provisioning: true

[Satellite-QE]

PRT Result

Build Number: 5730
Build Status: UNSTABLE
PRT Comment: pytest tests/foreman/api/test_discoveredhost.py -k test_positive_reboot_all_pxe_hosts
Test Result : ========== 14 deselected, 10 warnings, 1 error in 1422.19s (0:23:42) ===========

[shweta83]

trigger: test-robottelo
pytest: tests/foreman/api/test_discoveredhost.py -k test_positive_reboot_all_pxe_hosts
provisioning: true

[Satellite-QE]

PRT Result

Build Number: 5736
Build Status: UNSTABLE
PRT Comment: pytest tests/foreman/api/test_discoveredhost.py -k test_positive_reboot_all_pxe_hosts
Test Result : ===== 1 failed, 14 deselected, 11 warnings, 1 error in 2191.89s (0:36:31) ======

[Gauravtalreja1]

As I mentioned on original PR, PRT failure isn't related to the changes of this PR, but it is related to a BZ 2264195, I've opened a quick fix theforeman/foreman_discovery#623 in upstream already and handled the changes here.
And, now this PR is ready for review