Fixed expression evaluation for custom button in CustomActionMixin

[yrudman]

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1505988

MiqExpressions for Custom Action defined on Service class are built based on Service model. MiqExpressions for Custom Action defined on ServiceTemplate are built based on ServiceTemplate model.
Before:
CustomActionMixin did not have ability to evaluate actions defined on template or class differently. Expression were evaluated in the same way using instance of ServcieTemplate, regardless where button come from - ServcieTemplateor Service class.
As a result, evaluating expression for custom action defined on Service class was throwing error: NoMethodError: undefined method vms' for #ServiceTemplate:0x007fd4b6f74490`

After:
Use different object for expression evaluation: instance of Service for actions defined on Service and instance of ServiceTemplate for action defined on ServicesTemplate`

@miq-bit add-label bug, core

\cc @lpichler @gtanzillo

[lpichler]

@miq-bot assign @lpichler

[yrudman]

@miq-bot add_label gaprindashvili/yes

[miq-bot]

Checked commits yrudman/manageiq@d2716e0~...a9a8d4f with ruby 2.3.3, rubocop 0.52.0, haml-lint 0.20.0, and yamllint 1.10.0
4 files checked, 0 offenses detected
Everything looks fine. 🍪

[simaishi]

Gaprindashvili backport details:

$ git log -1
commit 039ca823711668e7b2ee4a553ce7a8ae644b3924
Author: Gregg Tanzillo <gtanzill@redhat.com>
Date:   Tue Jan 16 11:02:35 2018 -0500

    Merge pull request #16770 from yrudman/fixed-expression-evaluation-for-custom-button
    
    Fixed expression evaluation for custom button in CustomActionMixin
    (cherry picked from commit 091a546d55791f20d037d2d948308dc556d39d77)
    
    Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1535153