Resource controller and playbook bug fixes

[chungers]

This PR

• Fixes a problem with "" empty string in playbook's {{ metadata }} calls which cause the vars backend to be updated with empty strings.
• Renames the InstanceObserver.Labels to InstanceObserver.Select to better describe its purpose of providing a set of labels for query / resource object filtering.
• Improves the aws playbooks to use the vars plugin to store user-defined variables such as project name, CIDR ranges for subnets and subnet locations (azs).

Signed-off-by: David Chung david.chung@docker.com

[codecov[bot]]

Codecov Report

Merging #892 into master will increase coverage by 0.07%. The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #892      +/-   ##
==========================================
+ Coverage   48.46%   48.54%   +0.07%     
==========================================
  Files          84       84              
  Lines        7812     7814       +2     
==========================================
+ Hits         3786     3793       +7     
+ Misses       3718     3714       -4     
+ Partials      308      307       -1

Impacted Files	Coverage Δ
pkg/cli/playbook/modules.go	65.21% <87.5%> (+0.77%)	:arrow_up:
pkg/rpc/mux/server.go	47.91% <0%> (+5.2%)	:arrow_up:

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