Add connection-security constraint (issue #961)

[DimitriZhurkin]

Committer Notes

This constraint defines the list of allowed values for the connection-security property.

Related issue: #961.

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[Gabeblis]

Can we add the help url?

[Gabeblis]

This is good stuff! Just a couple small things that caught my attention.

[Gabeblis]

We should change this to http since we finally moved away from the https namespace.

Suggested change

	<allowed-values id="connection-security" target="system-implementation/component/prop[@name='connection-security' and @ns='https://fedramp.gov/ns/oscal']/@value" allow-other="yes" level="WARNING">
	<allowed-values id="connection-security" target="system-implementation/component/prop[@name='connection-security' and @ns='http://fedramp.gov/ns/oscal']/@value" allow-other="yes" level="WARNING">