Previously common-rst.pot is additive only and
deleted strings in the source document will remain forever.
This commit make generatepot-rst.sh not reuse existing
common-rst.pot and generate the POT file from scratch.
This patch creates an index rst file which includes all
files under common-rst and generate common-rst.pot.
By doing so, all contents of common-rst can be covered.
Closes-Bug: #1507138
Change-Id: I888f37ad5e14a0e1cc7d182e376acaf63f98e565
61b9f5a221