Enhance Eltex TAU model VoIP gateway fingerprints

[mkienow-r7]

Description

Follow-up to enhance Eltex TAU-* fingerprints introduced in #434.

• Add os.product for Eltex TAU HTTP server fingerprint
• Constrain model match to avoid matching non-existing models such as TAU-8.I, TAU-8.P, TAU-8.100
• Expand regex with serial number and sofia version allow matching models that include a letter in the model number such as TAU-32M.IP
• Expand serial number regex to include alpha characters since I located devices that included a single letter following the format VI3A###### and VI4D######.

Motivation and Context

Improved Eltex coverage.

How Has This Been Tested?

• rake tests
• ./bin/recog_verify

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• I have updated the documentation accordingly (or changes are not required).
• I have added tests to cover my changes (or new tests are not required).
• All new and existing tests passed.