Add support for pull-policies in Kubernetes stacks (v1alpha3)

[simonferquel]

Based on #1617

- What I did

Compose on Kubernetes now supports explicitly specifying a pull policy on a service. This PR enables support for that with the x-pull-policy field on service configurations.

- How I did it

Following the pattern introduced with pull secrets, I added a declaration of the hasPullPolicy capability, and pass whatever is contained in the x-pull-policy extra field.

- How to verify it

This is covered by a unit test

- Description for the changelog

Services in a Kubernetes stack can now specify their PullPolicy using the extra field x-pull-policy.

[codecov-io]

Codecov Report

Merging #1644 into master will decrease coverage by 0.22%.
The diff coverage is 54.92%.

@@            Coverage Diff             @@
##           master    #1644      +/-   ##
==========================================
- Coverage   55.89%   55.66%   -0.23%     
==========================================
  Files         299      302       +3     
  Lines       20479    20574      +95     
==========================================
+ Hits        11446    11452       +6     
- Misses       8210     8296      +86     
- Partials      823      826       +3