Updated epoch yaml syntax

[Matias Bordese]

Epochs (stepped upgrades) support is being reworked, and its syntax in the YAML will change respect to the original implementation (which wasn't really used).

Right now the reviewer tools are rejecting any revision with an epoch value that is not an integer:

The store was unable to accept this snap.
  - malformed 'epoch': 3* (not an integer)

However, the epoch field should start allowing (with the right yaml syntax):
  - an integer or a string, all digits, with an optional * suffix (eg. 1, 3 or 5*)
  - a dict with optional 'read' and/or 'write' keys, each holding a list of integers, non-empty and sorted (from lower to higher) (eg. {'read': [1, 2]}, {'write': [3, 4]} or {'read': [1, 2], 'write': [2]})
  - field is optional

This validation is also being implemented as part of the revision scan task in the store.

[Jamie Strandboge (jdstrand)]

This is now in trunk.