Better support for conditionals inside properties

Conditionals like IF can be used inside some properties, but because of strict type checking, troposphere does not allow for their use.

For example, the following is a correct template snippet (not a specific use-case I had, just an example):
```
"InstanceRolePolicy": {
"Properties": {
"PolicyDocument": {
"Statement": [
{
"Action": [
"logs:PutLogEvents"
],
"Effect": "Allow",
"Resource": [
"arn:aws:logs:*:*:*"
]
},
{
"Fn::If": [
"SomeConditionHere",
{
"Action": [
"logs:CreateLogStream"
],
"Effect": "Allow",
"Resource": [
"arn:aws:logs:*:*:*"
]
},
{
"Ref": "AWS::NoValue"
}
]
}
]
},
"PolicyName": "test",
"Roles": [
{
"Ref": "InstanceRole"
}
]
},
"Type": "AWS::IAM::Policy"
},
```

which should have a representation in troposphere as below:
```
iam.PolicyType(
"InstanceRolePolicy",
PolicyName="test",
PolicyDocument=aws.Policy(
Statement=[
aws.Statement(
Effect=aws.Allow,
Action=[
aws.Action("logs", "PutLogEvents"),
],
Resource=["arn:aws:logs:*:*:*"]
),
If(
"SomeConditionHere",
aws.Statement(
Effect=aws.Allow,
Action=[
aws.Action("logs", "CreateLogStream"),
],
Resource=["arn:aws:logs:*:*:*"]
),
Ref(AWS_NO_VALUE)
)
]
),
)
```

but it obviously fails with `TypeError: Statement is , expected []`

This specific example is related to awacs, but I imagine the same problems happen directly in troposphere as well, wherever the property is expected to be a specific object.

Should conditionals be allowed by troposphere inside most (all?) properties? Either via something simple to how `policytypes` are solved in `iam` or by something more complex which will detect the condition and check for the types inside the condition?

Start by reproducing the TypeError with the nested AWS IAM policy example and inspect troposphere's strict property type checking and If handling. Define the supported conditional cases and verify that conditional statements are accepted while the expected property types remain validated.

Written by the indexing model from the issue text.