Simplify options handling

[bear454]

• use case-insensitive non-greedy regex
• remove requirements/handler for quotes around options
• add an error handler

Resolves https://github.com/SUSE-Enceladus/blue-horizon/issues/155

[bear454]

Given a variable defined as:

        "deployment_type": {
            "type": "string",
            "default": "Simple",
            "description": "Select to deploy a simple server or a clustered environment with 2 servers with System replication and HA capabilities<!-- Options=[Simple,HA] [group:HANA configuration] -->"
        }

[codecov[bot]]

Codecov Report

Merging #158 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #158   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines          706       706           
=========================================
  Hits           706       706           

Impacted Files	Coverage Δ
app/helpers/variables_helper.rb	100.00% <100.00%> (ø)

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