Tags should support uppercase and slash characters

[thclark]

Feature request

Use Case

We need to tag met mast files with strings including '/' and capital letters, in order to tag with a timezone... eg

tags={"met-mast-id:mst1", "timezone:Asia/Kolkata"}

This raises the exception:

> octue.exceptions.InvalidTagException: Invalid tag 'timezone:Asia/Kolkata'. Tags must contain only characters 'a-z', '0-9', ':' and '-'. They must not start with '-' or ':'. 

Current state

The only way of working around this to encode an IANA timezone would be to create a shared key:value map of timezones where the keys are compliant with tags. This is gross overkill!

[thclark]

NOTE: From here we should be limiting to ASCII characters only to be compliant with gclouds XML api

[cortadocodes]

I've added this feature here: #152

All the allowed characters in a tag are ASCII, but not all ASCII characters are allowed in tags.