Convert trycmd() to oslo's processutils

trycmd() have been moved into oslo and is already imported in Ironic,
this patch changes our custom trycmd() function to use the one from
oslo's processutils.

Change-Id: I9df6d62d1729c3763d980b8ae38b399dce18e66b
Move and rename ValidTypes

Move the ValidTypes class from v1/utils.py to the v1/types.py file which
should contain everything related to wsme types in our API.

This patch also renames the ValidTypes class to MultiType because
it reflects better what this type about, it's a type that validates
multiple types and is more consistent with the other types names: Ending
with the word "Type" (singular).

Change-Id: I5ea5968bc3969f081bdf314771111e904462d195
Merge "Convert trycmd() to oslo's processutils"
Merge "Move and rename ValidTypes"