Allow setting of doc strings within the ini file

The @doc option can be used to specify the docstring of the command.

[cmd:nova.boot_test_infra_mgmt]
@command = nova.boot
@doc = Boot a test cloud infra machine with only the management network.

Change-Id: I4b1e28ad8f069bc06e57fbd1c0eb4b0c7a6738ab
Merge "Allow setting of doc strings within the ini file"