Introduce jenkins.model.JenkinsReloadConfigurationTest

(cherry picked from commit 140eaf07c669006433bc6c4dae8431e76458d01d)
[JENKINS-20772] Only show error message in dialog

This change removes the superfluous UI elements around the error
message if possible. Otherwise, fall back to existing behavior
of adding entire page to iframe.

(cherry picked from commit b856cc2434b33ed7fec3492223e141747369bba3)
[JENKINS-20772] Fix scrolling

(cherry picked from commit 9b947ae8a239023482b8dcdec97d37425c42be8c)