Adds storage gateway support, closes #6

[adam-imeson]

Issue #, if available: #6

Description of changes: adds support for launching file, volume, and tape gateways on target Outpost. Example tfvars file:

username = "imesona-sgw-test"

region_cloud9           = false
outpost_cloud9          = false
emr                     = false 
memcached               = false
redis                   = false
eks                     = false
eks_outpost_node_group  = false
mysql                   = false
postgres                = false
on_prem_vpc             = false
file_gateway            = true
volume_gateway          = true
tape_gateway            = true

allowed_instance_types  = ["m5.xlarge","r5.xlarge"]

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