Add setting to force install from MIRROR_HTTP on PXE boot

This commit introduces the setting *FORCE_INSTALL_FROM_MIRROR_HTTP*,
which when set to true forces `boot/boot_from_pxe` to install the SUT
from the URL contained in *MIRROR_HTTP* (grub boot `install=` argument)
even when running in workers with a closer mirror.

Idea behind this is to allow workers to specify a combination of
*PXE_PRODUCT_NAME* and *MIRROR_HTTP* settings which will lead to a
proper installation of a product/build under test.

tests/boot/boot_from_pxe.pm

@@ -126,7 +126,9 @@ sub run {
             $image_name = get_var('PXE_PRODUCT_NAME') if get_var('PXE_PRODUCT_NAME');
             $image_path = "/mounts/dist/install/SLP/${image_name}/${arch}/DVD1/boot/${arch}/loader/linux ";
             $image_path .= "initrd=/mounts/dist/install/SLP/${image_name}/${arch}/DVD1/boot/${arch}/loader/initrd ";
-            $image_path .= "install=http://mirror.suse.cz/install/SLP/${image_name}/${arch}/DVD1$device ";
+            my $install_path = "install=http://mirror.suse.cz/install/SLP/${image_name}/${arch}/DVD1$device ";
+            $install_path = 'install=' . get_required_var('MIRROR_HTTP') . "$device " if (get_var('FORCE_INSTALL_FROM_MIRROR_HTTP'));
+            $image_path .= $install_path;
         }
     }
     elsif (match_has_tag('pxe-menu')) {
@@ -185,6 +187,11 @@ sub run {
     die 'PXE boot failed, installation repository likely does not exist'
       if (check_screen('pxe-kernel-not-found', timeout => 5));
 
+    # Click 'Yes' if prompted to download a matching boot image
+    if (get_var('PXE_PRODUCT_NAME') && get_var('FORCE_INSTALL_FROM_MIRROR_HTTP')) {
+        send_key 'ret' if (check_screen('download-matching-boot-image', 420));
+    }
+
     if (is_ipmi && !get_var('AUTOYAST')) {
         my $ssh_vnc_wait_time = 420;
         my $ssh_vnc_tag = eval { check_var('VIDEOMODE', 'text') ? 'sshd' : 'vnc' } . '-server-started';