Required an update to the yaml package in order to use
unmarshalStrict.

Validate method currently runs and returns null. Based on the
issue description it seemed appropriate to always do a strict
check on the config and let the errors thrown there take care
of actual validation. The validate command is really just
a null method that loads the config like normal to perform
checks.

Ran into a few pitfalls and things to note:

 - Yaml is already coercing values so no need to validate "types"
   - int, string, bool
 - Regex seekms to require ^ and $ to denote string - might be go specific?
 - Keep validate command an empty stub and just exit the "Before" func
   - Doing anything else complicates the code with no advantages
   - Gains all validation entrypoints in 1 place

Making a lot of assumptions about validation rules for
structs.

 - Validator is using MathString hence requires ^$
 - Using types for contraints on lists of items is easier
 - Some validation is still loose
   - Validating an AMI just checks for ami-[0-9a-z]+
   - No good constraints on instance types - a list would be great

Trying to balance flexibility with usefulness. Contraining
to all current instance types that are valid would require
keeping mu up to date to support new instances - which seems
not helpful.

There were a lot of similarities between the regular expressions
I was writing and I honestly didn't like having regexp all over
types.go. The custom method names are more descriptive of what
the validation is actually doing and it means I can probably
just test these custom methods instead of writing tests for
each and every property of the types.

I already have tests for a lot of properties so I'm not going
to remove them. Going forward though I will only write tests
for the validation methods.

Added validation tags to the remaining struct properties. Also
added a few new types to validate properties that have limited
allowed values.

HealthEndpoint failing check in basic config because URLs could
not start with a / and required at least 2 characters. Requirement
loosend to allow alphanumeric and / for first character and to
only be a single character in length.

Was validating pipeline environment image as an AMI when it
is actually a docker image. The registry/image:tag regex
is fairly complicated and varies between v1 and v2
of the registries. For now just adding an empty method where
validation may occur later if we want.

Issue 258 - Validation

fix #258