Convert input values to str

If non-string values are provided to a software deployment,
os-refresh-config fails with:
  'TypeError: execve() arg 3 contains a non-string value'

Patch converts all values to strings if they should be strings.

Change-Id: Ifa442538da063e1f3034d02e9cc4f5ba6494206a
Closes-Bug: #1333992
Merge "Convert input values to str"