pyecmd: Handle const data types the same as non-const
pyecmd: Handle const data types the same as non-const

Strip the const qualifier from argument types so that when upcoming
changes constify, say, ecmdDataBuffer input argument, we still wrap
them into our conversion functions correctly.

The full argument type is retained in a new full_type property in
case it is needed later.

open-power/eCMD,open-power/eCMD,mklight/eCMD,mklight/eCMD,open-power/eCMD,open-power/eCMD,open-power/eCMD,mklight/eCMD,mklight/eCMD,mklight/eCMD