Extra_spec containing '/' can't be deleted

This patch validates the keys of the extra specs
before setting them.
This will make possible to remove those keys.
It allows alphanumeric characters, underscores,
periods, colons and hyphens.

Change-Id: Ic2e5d7ce42c8ecb7ee45cd168bd008035fd4bbdf
Closes-Bug: #1259711
Merge "Extra_spec containing '/' can't be deleted"